There are more considerations that just raw compression though. For example, running a gasket with a compressed thickness of .021 will put the pistons too close to the head, especially if you're running forged pistons and have zero decked the block. They'll be hitting the head at rpm and that ain't good. You generally don't want the pistons to be set up any closer than .035" from the head "cold".
Consideration two: you can't just "mill to 85 cc's" without first thinking about how much material there is in the head deck that can be safely removed. On Pontiac heads, .050 is about the max (and if you do that there's nothing left for "later" if you need to rebuild the heads to square up/re-flatten the decks). .050 will remove about 10 cc's from the chamber --- so on those 142's you can take them from 90 cc's down to 80 cc's, but on those 6x-8's you can only get them down to about 88 cc's.
I still don't know why your machinist says he can't install larger valves. 142's should be 1967 heads, and in 67 Pontiac changed the center to center valve spacing on all the heads specifically to accommodate the larger 2.11/1.77 valves. I could measure the center to center distance on a pair of my heads that for certain have the larger valves in them, and if the 142's you have are the same dimension, they'll fit.
If you cut those 142's "to the max" so that they have 80 cc chambers, run standard .040 gaskets, don't zero deck the block (Pontiacs are usually .020 "down" if the block is untouched so I'm making that assumption), then at +.030 with flat top pistons (that usually have 6 cc's in the valve reliefs) you'll be at 9.375:1 compression. That's workable. That Lunati cam you're planning is actually a little on the mild side (despite them calling it a "torque monster"). It should give you good idle vacuum, and the 112 degree LSA means you'll have a slightly later intake closing event that should help manage cylinder pressure and thus give you some protection from detonation, if you install it at their recommended 108 degree ICL. (By way of comparison, the 'advertised' duration on that cam is 262/268 - compare that to the Pontiac factory Ram Air IV which is advertised at 308/320).
With that "small" of a cam personally I would be hesitant to push compression any higher than 9.3:1.