after buying the wrong rims for my baer brakes, i need to do it again! anyone know how big a 17" rim and tire set up i can fit in the back end of my 66?

On my 66 Chevelle, I fit 295-50/15 on the back without modifications (no lip-trimming, etc... ). They are pretty tight fitting though, with about 1/8 inch clearance inside, and maybe 1/4 inch outside... Pictures in my signature below, but it used to be alot lower with the same tires...

Okay, so the tires are 15s, but check out a website like BFGoodrich, and look at the width/etc of my tires, and then find a 17 -inch near that size...

Then find what rim is suggested for that tire size...

I would think 17X9 or 17X9.5 would work (I think I have heard that before... But it depends on the offsets/back spacing available, not always adjustible...

What size wheel and backspacing do you have now,George? I would think a 17x9.5 or 17x10 wheel with 5.5" backspacing would fit your car,,assuming the body is on the frame,squarely.
Either a 275/50/17 or 285/40/17 tire will work on the above width rims.