I've have put heads and manifolds on GMC M/Hs 4 times - here's what works for me

1. Before the heads are removed
   a. It is a good idea to turn the engine to TDC and measure the lifter preload for a couple of cylinders - Lifter preload is how far the pushrod cup in the lifter is compressed below the  snap ring when valves are fully closed (like TDC ) - this distance should be about .030" - .050" - I believe that you will find that if you back off the rocker a couple of turns and then re-tighten till  the rocker arm just makes contact with the valve stem - it will be about 1 full turn further down when the rocker arm is fully tight (IIRC - measure with a feeler gauge first - then check rocker arm bolt turns) - give this info to your rebuilder - (see section 2)

2. After the heads have been removed
       A. Measure the distance between the head bolt boss and the deck  surface of the head. the average stock measurement is 2.200" to 2.220"
       B. If the heads have been - or will be surfaced - more than .030" (.050 is about the max limit for our heads). Than choose one of the following. remember most replacement head gaskets are almost that
       much thicker than the OEM gaskets so you have that much built in lee-way(sp) - The OEM shim style head gasket would crush to .017" and the standard replacement Felpro crushes to .043"
          1. Mill the surfaces of the intake manifold a total of 1.3 times what ever was cut from the head.  ex .050 milled from head  would indicate that you want 1.3 x .050" = .065" milled (decked) from the manifold
      2. Call Mondello and try to find a head gasket set that will crush to a thicker dimension to compensate for the heads being milled greater than .030
    C. Lifter preload is an absolutely important factor - - Your rebuilder needs to understand this completely - Mandello sells a gauge HG-455 to check this clearance (if your heads have been milled over .030 then this must be compensated for with the Mondello gauge).   There are other way to check for this clearance - most rebuilder have a gauge that will measure the valve installed height - and   with the information obtained in step 1 - the rebuilder can calculate what the installed height needs to be to get the lifter preload between .30" - .050".

3. Rebuilding the heads- here's what I did
        I made sure they used all NORTH AMERICAN parts  (Fed Mogal - Speed Pro - same co. -- good solid stuff)
     A. Had the heads Magnafluxed(sp) for cracks
     B. Made sure the heads are not warped
     C. mill only the minimum amount off the surface of the heads to get a smooth surface
     D. milled the exhaust side - minimum for flat surface
     E. Had only the INTAKE valve bosses cut to accept positive valve seals
     F. I had my heads "POCKET PORTED" - which removes most of the imperfections in the passages and enlarges the cambers for better flow - this is one of the few things that directly effect LOW END torque. I believe they used a chevy "J" cutter to do most of that work.
     G. Installed "thin wall Manganese/bronze liner" valve guides
        1. intake valve guide clearance to minimum .0020" - .0025" max.
             a. - Mitrol(sp) positive valve seals
        2. exhaust valve guide clearance to minimum .0025" - .0030" max
           a. - OEM umbrella valve seals (for extra lubrication)
     H. Measured valve springs & replaced if neccesary
        1. check rotators and replace keepers
     I. Used the largest intake valve possible for MY  engine - if you have 455- 1.995" try to get 455 - 2.072"   your rebuilder can help you select what will work
     J. Use valve seats inserts - ONLY IF ABSOLUTELY neccesary - and if you do - make sure there OLDS seats - not chevy seats
     K. 3 angle valve job
     L. After you get your heads back - place a steel ruler across  the tops of the valve stems - they should all be within a  few .001"s

4. Installing heads
     A. prelub the pushrods
     B. check preload (shim if too much - oversized pushrod if too little (this should not happen - if your rebuilder did it right)
     C. I used Felpro header exhaust gasket because they had the max   amount of contact area - I coated them with "NEVER-SEIZE"(sp) so that they would be more likely to slip when the manifold cools
        down and moves - (re-tighten 2 more times - each time  after the engine has been allow to cool down over nite)

5. Installing Intake manifold
     A. I used Mondello block off plates - his kit contains 2 sets   because the cross-over port came in 2 different sizes
        I do not believe anything else is required - no furnace  cement - no alloy -no metal insets - The only thing you must do  is find an electric choke

        I paint my intake manifolds OLDs blue - and as long as the paint stays a nice pretty blue - I'm assured that the plates are holding - I have many thousands of miles on both engines I've blocked off with only the plates - and the intake manifold is still ----unburned

     B  I use the OEM valley pan and cut out a space big enough for  the Mondello Block Off Plates to fit. This prevents  any doubling of gaskets
     C  I use Blue RTV (sensor safe - in case you have O2 sensors) and apply a bead around every intake port  and water jacket hole. (Zeb Fredy has used this trick to correct many newly rebuilt  engines that were using excessive amounts of oil. Oil that was  being sucked up thru the valley into the intake ports.
        (thrust me it works great)
     D  I do not use the supplied valley end gasket (rubber or felt)  because they can hold the intake manifold from seating properly   I used a heavy bead of blue RTV and let it skin over (about a half hour) - then I believe that most intake manifolds have 1 bolt hole that is a slot - I put a long bolt in the head &
        use that bolt as a guide to lower the head down with 2 hands. this methods lets me put the intake manifold on with a minimum of movement of all that RTV stuff.
     E. after everything is tighten down -- go inside a come back the  next day -DO NOT TRY AND START THE MOTOR - the same day - you may blow the RTV out the valley edges - wait the one day - you will not be sorry - this gives ALL the RTV a chance to work its wonders.
I need to give credit to
  Dick Paterson
  Brent Covey
  Zeb Frady
  Joe Mondello
It was the combined input from all of the above.  -- Thank
You do what you want - I just know what works for me  I just want to prevent anyone else from falling down the holes I already feel thru - isn't that the  whole purpose of this group

If I've missed anything - please feel free to email your corrections
Pete - Head nut here in the asylum