On the rear shackles, the later Chevy ones will get you metric size holes, I have a set and decided not to use them for that reason.

I had my leaf springs de-arched instead.

I think, however, that the earlier, early '70s GM stuff used the american, sized standard bolt sizes, same as Dodge.

But, I didn't buy any just to look at the holes ...

Only guessing, but probably '70 - 72? would do the job, and also thinking the '73-87, same same.

Either of these are only going to get you about 2" though.