Will Original temp gauge work with big block?

Yes. Don't overtighten, you can ruin the "guts" I don't recall if slants had a different size. They are common pipe thread, but if you must buy one, any "real" parts store should have 'em and they fit about 20 years. All through the 60's and 70's at least

Rock Auto shows SMT (Standard) TS-17T is a whole 4 bucks LOL

Summit wants 15 bucks for the same thing!!

The V8's are 1/8 pipe which is approx 3/8" OD I believe the slants are larger, perhaps 3/8 pipe? Which is about 5/8 OD

A quick search on th' bay shows lots of them under 10 bucks