An easy way is to stick something like a straight piece pf coat hangar into the head bolt holes, then measure to see how deep it is. To that add the thickness of the exhaust flange and gasket. That'll give you an idea of the overall depth you have to work with. From that figure subtract 1/8"/1/4" to prevent bottoming.
I'd measure all of them just in case one or more are shallower than others.
He wrote "header gaskets" AND "exhaust manifold", so I tried covering both bases just in case. After examining stock LT1 exhaust manifold gaskets it was obvious to me that GM put some serious engineering and thought into them.
One thing I forgot to mention is last summer when I was installing new Dart heads on my son's 96 LT1 Vette, I had to do some pretty extensive cutting and grinding to get the stock exhaust manifold heat shields and a couple of spark plugs to clear. That's one reason I mentioned possible clearance issue(s).
Keep us posted.
My son, Ryan M. Cameron, graduated from West Point on 22 May 2010! He was commissioned as a 2nd Lieutenant and I pinned on his first pair of "Butter Bars" PROUDEST DAY OF MY LIFE!!
1996 Vette, 388 CID LT1 (3.75" X 4.060"), Eagle Internally Balanced Forged Crank, Eagle 6" Forged Rods, SRP Forged Pistons, CompCams XFI 467 Cam (230/236 @ .050), Scorpion 1.7 RRs, AFRs ported to 212 ccs (304 cfm @.600), Match Ported LT1 Intake, SVO 30# Injectors, BBK 58mm