you want the 83 L52 tranny, the other one was junk
The only difference between the L50 (1981-1983.5) and the L52 (1983.5-1983.12) is that the L52 has a 5mm oversized input bearing (75mm compared to 80mm). Everything else is pretty much the same internally and externally.
it's a direct bolt in for the most part. you'll have to grind down the splines on the t-case, since instead of having a relief area on later cases, the 4 speeds had raised splines past where they really matter
Also you may have to hammer/clearance the tunnel a little bit because the top shifting 5-spd L-series is slightly taller than the 4 spd..
if you get a later tranny, you'll have to get new driveshafts made(or find a suitable donor), and relocate your t-case crossmember.
The 84 and newer G, W, and R-series are 4.5" longer.
The ultimate L-series is the L52HD
that Marlin makes. It uses a different gear set with the same ratios that is about 30% stronger then the L 45/50/52.