I'm new to the early chevy trucks but I think that 4 speed you have is a sm420??? correct guys? If that the case then it is a very strong tranny. Much stronger and more reliable than your th350. I went thru a th350 a year in my wheeling truck...but since I switched to a sm465 (the newer/weeker version of the sm420) I have yet to hurt it. Its also stronger than any of the car trannys out there....
But there are drawbacks. First gear (granny) is a 7:1 and isn't sincroed. There is a big gap between 2nd and 3rd. Plus its a truck tranny so its going to have to be shifted slow compaired to a car tranny. I would also upgrade to a bigger clutch. The 2 420's I have sitting here have clutches much smaller than the one I use in my wheeling truck.
