I am about to purchase 15 x7in (110mmx4) steel wheels with 4 in backspace from Diamond Racing, but they are non-returnable. Will these size wheels (with 205/50 HR 15 tires) clear the springs and fenders?

 

4" backspace is roughly +1/2" offset. In metric terms, that would be about +12.5mm.
I run 15x7 wheels with a +15mm offset, along with 205/50-15 tires, and they clear the springs fine. I had to roll the fender lips, but my car sits quite low.

I'm pretty sure the Diamond wheels have been proven to fit rather well over the years, or they wouldn't reccomend that size/backspace combo for this type of car.

 

I have wheels for a 1st gen with a +15 offset in the rear....15x7....plenty of clearance. 205's will fit no prob. YOu can get a 7.5 or 8" wheel in the back with proper offsets.