Transmission overhaul or upgrade???

Does anyone have an opinion on whether I should overhaul my light duty SR 4 transmission or should I upgrade to a SM 420. The SR 4 pops out of 2nd gear, other than that it works fine.
I know where a 65 Jeep is with a SM 420 in it that I can get for $500.00 (the whole jeep with no engine)
This swap would be for a 1981 CJ 5 with a GM 151 engine and a Dana 300 transfer case. It also has the Model 20 rear axle (upgraded to one piece axles). Will I need any adapters etc. if I choose to go with the SM 420?

Thanks

 

me
I'd rather fix what I already own rather than buying someone else's problem..
that being said, you don't know what the SR4 is gonna need till you take it out and tear it down...could be just shift fork....could be a worn out mainshaft...
there are some other tricky shift components inside the SR4 top cover...
I still say find out what's wrong with yours..it might be simple.

 

Why not get the 65, and swap everything into that?
Then build something more aggressive with the 81.

 

The 151 is not a powerhouse - I'd think it's not that bad a match to the SR-4. I'd certainly follow Jim's advice and look inside before I made any big moves. You could probably find a replacement SR-4, T-4 or T-5 on POR or your local 4x4 forums for short money. Also, a lot depends on what you want the Jeep for. If you plan to abuse it, then the SR-4 won't last.

 

My vote; SM 420, one of the best tranny's out there.