ok finally got the old slicks off these cragar Super tricks 15X8, 4 5/8s BS, placed them on the drums....plenty of room to spring, not a lot on outside lip, see up higher in the wheel well as these is some kinda bulge in the sheetmetal, what do you think, can i use these? What size tire? Can i put a smaller tire on a wider rim and squeeze it into the wheel well or do i have to start over with different width&BS rim?

more photos

passenger side

front shot pass side

I think ( it was a while back and I am not getting any younger) I used 5 1/4" backspacing on my rear of my 67 with 255 or 265 60 series

I had a 15X7" OEM steel wheel with 4.25" BS on my 67 Belvedere. Tire size was 275-60-15. I could barely squeeze it into the wheel well. I had to disconnect the shocks so the housing could drop a bit more just to get in in there. They were too much of a hassle so I took them off.
I don't recall how much room there was between the tire and spring. The 4.625" BS on your wheel may help a 275-60-15 fit better.
When I get the car back on the road, it will have a 255-70-15 on the same 15X7" wheel rear, a 225-70-15 on a Torque Thrust 15X6" front. A 255-60-15 tire would also fit nice on the rear but I need a tall tire to make up for the 4.88 gear (till I get it changed to 4.10).

My 65 coronet has 15x8 with 4.5/8 and I have 295/50/15 on it with SS springs and I could go bigger. the lip of the fender is 2 inches above the wheel.
No mods to the spring perches.

65 has bigger wheelwell opening all around ,66 is orphan when it comes to the opening, i have a 65 next to my 66 and you can see the difference.

65

65s seem to have a more flared opening,than does the "slab side"66