Early trans were iron case 18 spline,modified to slick shift/crash box.
Later years were aluminium case slick shift/crash box.
At some point they tried Borg Warner T-10 or "rock crushers" since NHRA was letting offbrands use the aluminium Chrysler boxes.
The class progressed to any manual shifted transmissions,OEM or aftermarket.
The maintance on the slick shifted trans was tremendous,mandating carrying quite a few spares for a weekend of racing.The weak part was the imput shaft and the stress when 4th gear was use since every other tooth was machine off.This was true to all the other gears and the sliders but worst on the imput.
With the production of new "clutchless manual transmission" and the tech. today those trans mods back in the day dinosarous and should be left to the past unless your a Sox,McCandless,or Jenkins and have a big bankroll and love servicing a 4 speed constantly.
I got rid off all my 4-speed stuff and even buried some aluminium cases out"behind the Hogpen",Bring your metal detecter and a shovel and I'll point you in the right direction.