The main difference is the output shaft in the transmission is longer if it’s designed for the overdrive
I think the rest of the box is similar with the addition of the overdrive
I added one and shortened the first half of the driveshaft in my Amazon and run dot 5 silicone fluid not 3 or 4 in the clutch
I switched the brakes to dot 5 and it works ok
I just park it lots and didn’t want water in there or paint issues from the dot 4 or 3

If you drive it hard your brakes might be a bit mushy or may boil
The clutch isn’t an issue I’ve found the rubber seals last way longer particularly the clutch ones

Finding the trans to use might be hard

Later 122s about 69 or so used the cable I think.. 67 was still hydraulic

The cable means you don’t need to deal with leaky master and slave clutch cylinders
I went through rubber kits like crazy then swapped clutch to silicone and it went 20 years without a failure