Welcome to CHR! We're glad to have you here, and hope you'll stick around through getting that Chevelle back on the road and more. Like Glenn says, you really shouldn't even consider a flat tappet cam, because the changes in oil additives (took out the zinc) make them unreliable. My engine was built at 10.3 to one compression and needs 91 octane to be happy, and I sincerely wish that we had cut it back to 9 or 9.5 to one. You'd be surprised how many places don't have three tanks any more, and you never know how long that premium has been sitting in their tanks, either. Lastly, I know it's one of those Memory Lane trips to say "Yeah, I've got the double hump heads with the big valves....", but head technology has come a long way in the last thirty years, and aluminum heads help you dissipate heat soooo much quicker which will help you in that compression ratio arena for one thing.
I second Glenn's advice that you at least consider a turn key crate engine for your Chevelle. Unless you're wanting a numbers matching, all original, deep pocket restoration targeted at a serious collector I think you'll be happy with what you find. You can always keep the OEM stuff in the back room, and change back if you want the numbers matching thing.

And to close,
Quote Originally Posted by glennsexton
If you’re set on building an engine, you may want to also consider a 383 - but definitely build with a hydraulic roller cam. Tons of build knowledge here from Pat McCarthy, Jerry Clayton and others who are more than willing to share with those who genuinely want to listen and learn. Tell us a bit more about what you end goal is, i.e. cruiser, daily driver, bit o’ drag racing and what you’ve got in mind for budget.

Regards and once again, welcome to CHR.