Valve job on os valves

another point where there's lots of info on the site about. Look around, even just within the /6 topic area.
On mine, I used an Aussie head gasket (0.060" out of pkg, 0.050" compressed-- actually measured, "my68barracuda" pointed out a spot to me, at back of head/block with a little room for the edge of feeler gauge blades so you can measure what your head gasket ends up at, while compressed) Many are listed by various members right here. so you can get a pretty good idea from reading the posts where yours "should" fall.
and 0.100" combined, shaved off the head and block/ I wound up right at 8.4:1/ I was hoping for a little more. I had a thread not too long ago about "that came up short". With my particular head gasket I had to run (for other reasons) and its larger fire ring, plus its added thickness the 30- thou I had taken off the block "just" makes up for the added head gasket thickness, vs how it was at the factory. It doesn't help that most "rebuilder grade" pistons actually come up short of where the original pistons sat, "in the hole" at TDC, mine are just about where most guys find theirs on a stock unrebuilt engine, even with that 0.030" I had taken off the block.
I don't know that I could measure the actual deck height on my engine, that may be a "machine shop guy's" job. but it is known what the design spec was supposed to be for deck height as built brand new back in the day. But even considering all that, at the end of the day that 8.4:1 CR is better than my engine ever was from new, by virtue of all that milling/machine work. It just sucks to have had to do "all of that," to even get to "design spec". on my particular combo, that means there was an extra 0.070" "somewhere," that wasn't supposed to be there. though granted, I did use a newer "peanut" head, I want to measure the original head's chamber size, and plug that into an online CR calculator. That original head hasn't as of yet, been milled or machined at all, since new.

Depends on assembly line tolerance stackup, how tall your block was when new, and actual size of chamber. They seemed to vary alot on these engines, over the years. All of the old school auto repair manuals would have a person working on their car believe that most all /6s came from the factory at 8.4, but most were really truly in the 7's to 1. You're gonna have to make a few measurements off of YOUR engine, to see where you stand now. plus who knows who has been inside of that engine, over the years?

That 40-thou head gasket will crush a bit, 40 isnt the "installed thickness. Id guess it would end up at 30-35 thou. (wont mean THAT much at the end of the day)
but you need to know where YOUR pistons sit at extreme TDC on YOUR engine, (how far below deck) as well as to measure CCs on YOUR head that you are gonna use on it to figure that out. I was hoping to be closer (but not quite "to") 9:1.