Wellllll...... if I run the numbers
83 cc chamber
4.16 gasket bore (Fel-Pro Q8518PT)
.016 deck clearance
5 cc valve relief
I get 8.170:1
With an .042 gasket, .020 deck, and 6 cc's of valve pocket it comes out to 8.092:1
Keeping the .042 gasket, .020 deck, and 6 cc's ---- you need 76 cc's of chamber volume to get to 8.613:1. Using .045/.016/5 it takes 77 cc's to get to 8.622:1
(All figures come from using the Excel spreadsheet that's embedded in the attached Word document)
One thing you might try... instead of dumping the whole 50 cc's at once, don't use the whole syringe capacity --- work between two discreet markings on the syringe and dispense/refill it say... 10 cc's at a time or something, just in case something's not marked right.
Searching the web for reference material, it seems that there's some discrepancy out there with regard to what volume they were, nominally. I found them listed at 73 cc's, 72 cc's, and several other sites that just said "8.6:1" which was a little unusual. I do know that Pontiac heads can vary from the factory specs, and the factory specified "compression ratio" especially was always misstated by quite a bit, but the difference you're finding does seem to be excessive (depending on which, if any, web site is 'right').