Why not call Hotchkis and ask them for a baseline recommendation for their kit?
No matter what you have a 44 year old car and they are all a little different so you need to measure your car to ensure you get the right fit.
A 20" wheel is pretty tall in the first of the first gens and not a lot of people run a 20. If you want the stuffed look a 18/19 will get you there JMO.
From misty memories of stuffing 16x8s in a 69 Firebird in 1981 I'll say no way a 10" wheel will fit with stock springs. I could be wrong it's been 30 years.
