I'm currently using pretty stock 15x6 wheels with the full cap. The car has been lowered, and they fit well.
But I also tried some spare 15x8 wheels I had and 275/50 TAs which I used to have fitted to my '70 Galaxie. First thing I noticed was that from the stock 215/75s to the now 275/50s, the car dropped about 1 3/4" closer to the ground.
I took a few measurements.
Front and rear 15x8 rims have a 4" back space.
Rear: There is 1 1/4" between the tire and the leaf spring.
1 1/4" from the tire to the lip of the rear wheel opening, but a little further up inside the wheel well, the clearance is just 5/8" to the inside of the quarter panel.
Front: With the wheel in the straight ahead position, there is only 1/4"-3/8" clearance between the tire and the top ball joint. 1 1/4" clearance to the lip of the wheel opening on the fender. There is plenty of clearance from the outer tie rod end.
Based on this info, on the rear you could possibly go for 10" rims for better fitment of the 275s, but keep the clearances to what I have given. I think 295s might be a little too close for comfort.
On the front, I'd stick with 8" wide rims, because of the tight clearance to the upper ball joint.