318 Year range best for machining

You can get rebuild parts for the poly 318, Egge has pistons and kits for them.

But, I wouldn't build the poly for a '67. It'll end up costing more to build than a LA 318, plus its harder to find speed equipment for them (headers, intakes, etc). Again, that stuff is out there, but it costs more.

For a mild LA 318 build you shouldn't have any problems as long as you're going less than .060" over. Some of the LA 318 blocks have been known to go .080" and .090" over without issues, although you might have to sonic check a few blocks to do that.

If you really want to get tricky, look for an '86-'91 318. Those are the years you'll find 302 heads and factory roller cams. Not all of them were set up like that so you do have to check, but they make a great start for a build.