Flycutting cast pistons.

I wouldn't cut the pistons. You won't be able to deck the block that much, I think the most I've heard of anyone cutting one was .030". Not to say you can't take more, but keep in mind if you cut that much you're going to need new pushrods, and may run into valvetrain geometry issues if you really start cutting on things.

How much compression are you looking for? 9:1? 10:1?

Have the chambers on the heads cc'd. The 675 heads ran smaller than some of the later ones, usually they're around 60-63cc's. Once you know the size of the chambers, you can start figuring out what your compression will actually be. Then you'll have to figure out how deep the pistons are in the bores. My '71 318 measured out with the stock pistons .090" down in the bores, I've heard of as much as an 1/8" (.125).

But, even with the stock flat tops, if your chambers are close to the actual spec (60cc's), you can run a thin head gasket (mopar is now making a .024" thick, 4.060 diameter head gasket, Mancini has them). Even with the pistons at .090" in the block, with a 60cc chamber and a .024" gasket the compression should be around 8.9:1. Not super, but not horrible. Shave a few thousandths off the heads and you can be at 9:1.

But as I mentioned, keep in mind as you cut the deck or the heads that if you make significant cuts, you're going to need new pushrods. Not sure about there, but a custom set of pushrods costs around $200 here. A new set of KB167's, which have a compression height of 1.81" vs the stock 1.741", runs around $277. And unless you're getting the machine work done for free, when you add the price to deck the block or shave the heads, you might actually be money ahead just buying the new pistons.