Hey Mike, I'm running a mild 1835 w/about 90+ with a stock 4 spd. That's obviously less than what your pushing but mine gets along pretty good. I've taken her to about 97mph with no problems.Mines a 74 but I'm not sure what year the tranny is out of. A have trouble with wheel hop in 1st if I stand in her too hard. But I understand the tranny straps you can buy can alleviate that. I think all in all as long as you don't abuse it it should last decently I would think. I'm guessing with 120+ horses it is probably not going to be a daily driver? If that's the case, I'd say easy does it and get you one a little heavier to keep on the back burner if/when you need it. What have you got that your pushing 120+ out of? Just curiuos, I'm in the process of building one for a bus and I have another that I want to go all out on to replace the 1835 when the time comes.
'78 Riviera camper
"..at least I'm enjoyin' the ride"