Any equipment from the 50's on will use the flat back, as for size 33" for 70 ton & less cars, 36" for 90 ton & above except the 125 ton cars which used 38" wheels. Simply put, any questions, ask. To determine the ton rating look at the CAPY on the car data & divide by 2000 i.e. 140000 lbs divided by 2000 = 70 tons.
Addendum: I guess I should have said I use Proto wheelset where ever possible, freest rolling ones I know, but they don't fit in all trucks, there are different axle lengths between Proto, Kadee, IM & JayBee, I know the Protos didn't fit in the RedCaboose trucks on a car I built a few weeks ago, too long, I used Kadee. Just an example.