At least at the NHRA onset of Pro Stock a lot of the (lesser) cars were from the Modified Production classes and a few from Super Stock. But the Super Stock cars needed more modification. Back then you had to retain the original floor board though a cage was welded in to reinforce that structure as well as provide safety, and I believe both those classes required that. Gas class cars I recall being allowed greater frame modifications. Modified Production cars were allowed dual carbs where as the NHRA gasser were allowed injection. So, I can't say a gasser never became a NHRA Pro Stocker, but other cars were far more readily adaptable. AHRA and IHRA might have used different terms prior to the NHRA Pro Stock classification
. Most all the early cars were Barracudas, Challengers, and a few Darts (again the upgraded SS cars), Camaros, a few Novas, Mavericks/Comets and all of them big block cars. The small block Pinto, Vega/Monza, Duster/Demons/Arrow/Sapporo did not show up until the refactoring I'm thinking around 1972 and later.
Those interested might delve into "The Dawn of Pro Stock." I think the differentiatio
n is where gasser like cars went towards funny cars and super stock cars upgrade to pro stock cars.
https://books.google.com/books?id=NC6QUIR0TN0C&pg=PA6&lpg=PA6&dq=the+dawn+of+pro+stock&source=bl&ots=C9KqdI7Joy&sig=mgNbbWxzplbaQG0D4txjsuXtuI4&hl=en&sa=X&ved=0CGYQ6AEwC2oVChMIg6fhldOhxwIVAk6ICh0duAzW#v=onepage&q=the%20dawn%20of%20pro%20stock&f=falseAnyway, by the time the NHRA got around to Pro Stock there were many purpose built cars that dominated. With Chrysler it was Sox & Martin, Carlton, Landy, Mike Fons (Rod Shop). With Ford it was pretty much Nicholson and he wasn't all that competitive. With Chevy it was Jenkins, Booth (early), Shepard and Lombardo. With Chrysler so dominate they changed the factoring that made Jenkins Vega very competitive and after that came such competitive oddities as the Gapp & Roush 4 door Maverick and Wally Booth's Gremlins and Hornets. Through all this Glidden just turned it into pretty much a one man show. My focus went elsewhere and the next thing I know Pro Stock cars are like funny cars with carburetors. I never got the interest back.
I just read that Pro Stock will run fuel injection for 2016. This should prove interesting - I'm not sure exactly how, but interesting.