I know this subject has been beat to death and I have spent days reading threads on wheel sizing and backspacing. many thanks to everyone and especially @George P for his super useful data and comments.
My car is a '73 with Ridetech coil-overs and is lowered a bit. In lowering it, I snagged the front wheel arches with my stock 15x7 and BFG TA 205/60 tires, so fender rolling is on the to-do list regardless (as is raising the car back up a little).
The wheels I'm looking at are the AC-III Cobra/Halibrand from ET:
For the fronts, 17x8 with 5" backspace with a 225/45ZR17 (25" OD)
For the rears, 18x11 with 6.5" backspace and a 285/40ZR18 (26" OD)
Or if I went with Falken Azenis, I could use a 295/40-18 which is a little taller at 27.2.
Thoughts on tire sizes and backspacing would be very much appreciated.