For the power level you want a 350 Pontiac engine would work just fine. There have been lots of Pontiac 350's for sale that were junked because nobody wanted 'em. Some guys couldn't even give 'em away. But, for your power level, a good 350 shortblock, with the 670 heads should make a a decent engine.
For the big valve heads, you'd need a block with 2 valve chamfers at the top of each cylinder, for valve clearance/unshrouding. I've read that many of the 350 blocks from the '60's & early '70's do have both chamfers. But, some blocks have only 1 & some blocks have none. Chamfers can be added by the machine shop, or anyone with a small hand grinder, who knows what to do. The only 350 blocks to avoid are the 500810 blocks. They have thin webs just like the 557 400 blocks have.
And, the cheap replacement pistons have only 4 valve reliefs, like the '68-'73 originals.
Without checking the numbers, most would not be able to tell whether you had a 350 or 400 block in your car. You may be able to find a 350 for a very reasonable price. I've actually seen ads offering 350's for free, with pick-up.
Hey, the 350 is just one option. You can even stroke a 350 block, if you want more power.
