355 w/ 4 valve relief flat tops 64cc camel heads Xe 268 cam. I am assuming block and heads are stock and have not been decked.Is there any other info needed.

Probably right at 10 to 1 as a guess.

Try here;

http://www.rosspistons.com/calculator.php

Input 4.03, 3.48, 64, .039", .025", -6 and finally 8 and enter..

It will be pretty darn close!!

pdq67

I assumed the block has not been decked and the pistons are sitting at a stock deck height of .025" in-the-hole at TDC. I further assumed a typical FelPro .039" rebuilder-style head gasket. You'll be a smidge over 9.6:1 if all that's true.

It's a compression ratio that's pretty well matched to your cam. You should have no problems on pump premium, maybe even 89 octane if you pull the timing back a little.

Cool, i run the timing at 34 total @3000 rpm without the vacuum advance,and i also run 93 just incase. I have been debating a head and intake change and possibly cam if needed or just a crate motor.The bottom end does not have that many miles on it.I would like 400hp or better. I have been eyeballing some 383 crates on ebay for 3500 free shipping seems hard to beat.

Yeah that's between 9.5 and 10.0 considering deck height, piston maker, etc. etc.

My setup is about the same and I am at just around 9.6:1.