I used to do lots of hi way driving in my old van, I went through this exercise about 15 years ago.
Removed my M21 & fitted a Supra 5 speed behind the 308 with a 3.08 diff, 2k dollars later had myself a real hy way cruiser.
12 months & about 80,000 ks later it tore the cluster gear clean out of it. (about 400k north of mt isa near the Barkley homestead) In the middle of nowhere.
Got a replacement box $1200 sent to Isa from Townsville fitted it myself 7 continued home to the Gold Coast. All good mate. 12 months or another 80 odd thousand k's later,
Exactly the same thing happened.
You would not give me one of those boxes now, I hate them. My car was never used to do burnouts or crap & it has always been well maintained, I needed it to be reliable.
After much thought & advice we ended up putting a turbo 700 in it, (the first auto i ever owned) At the time the transmission was worth more than the car,lol. But i never had a problem again. (100k/r 1300 rpm) converter locked up.
Now the old girl sits in the shed, with the 3rd motor in it, waiting to get taken back to Mandarine Red & get an m21 back into her.
Just my 2 cents worth,
Cheers Vic.