So my Cuda has 15x15 wheels with 5" back spacing. It has 31x16.5 hoosier DOT pros on it.....

I want to put a set of 315 drag radials on it with a 12 wide rim. My calculation says I only need 2" of back spacing....And now I can't find any rims with that spacing only 3.5" BS....

New rear I’m afraid. Really hate the tucked in deep wheel look (5” between tire and fender)

I believe RC Components has a 2" backspace rim. They are pricey but they are nice.

That is correct if you want to take all of the width off the backside of the rim. You can pull the whole tire in close to the center of the car by splitting the difference. It is really just about looks since performance should be the same.

Holeshot offers 2.5" BS on 12" wheels

So is my math correct?

Yes, the 12" wheel with 2" BS will be the same distance to your fender as your current wheel.
If you want the wheel more tucked in get more BS just don't get black wheels

I like those, and price is competitive....I might go with those because if I add a bead lock, I want to be sure to clear the fender

If you go with beadlocks you might want to contact McFab. They can add a 1/2" to the front or rear spacing. When I did mine I added 1/2 to the rear, wish I had added 1/2" to the front also. My rims are now 12.5.i run 29.5x11.5w. it would flatten out the sidewalls a little.