i have a 77 olds 350 with 3a heads on it. do the 7a heads swap over with no problem?

 

YES no problem with the swap but you are dropping the compression and I could tell the difference right away, no scoot. sorry I did it in fact am doing a set of number heads now to put on my 75 block to up the compression.

 

No, the 3a heads should be about 77cc. the 7a's about 68, therefore you'll be raising your compression not dropping it. Put bigger valves in the 7a's and the swap will be that much more beneficial.

 

You will need to open up the head bolt holes. All 1964-1976 engines used 7/16" head bolts. 1977-1990 used 1/2" head bolts.

 

To add to the above post, most guys go a little over, either 33/64" or more often 17/32"

 

Related question: Is it possible to shave enough off 7a heads to get a '76 350 up close to 10:1 without having to change pistons? If so, would this require both adjustable rockers and adjustable/custom pushrods?

 

Nope, unless you can find shim head gasket. The max I have been told that you can mill that head down is to a 58 cc chamber. I did mine to 60cc. 14 cc dish, 58cc chamber, .028 gasket assuming pistons are .025 down gets you 9.5. And, milling of the intake and different push rods are required. I am not sure how much you can cut off a 330 head.
Now, if you pull the pistons and have the deck cut to achieve .040 piston to head, you are getting very close, like 9.9 to 1.