I just got a '70 K5 4X5. It has a rebuilt small block 400 w/ a Holley 600, little beefier cam, and I think the heads have been mildly ported. It has 31 inch tires and 4.10 rear gears. It HAD a TH350, until my dad took it for a spin to show my uncle what it could do. He was standing on the gas pedal and manually shifting the trans, but when it he shifted into third, it broke. My dad's a mechanic so he can replace it himself, but my question is this: will a rebuilt TH350 w/ a kit like TCI's Truck master rebuild kit be strong enough to handle the torque the engine is putting out, or does it need a TH400? My dad thinks it needs the 400 because it would cost more to build a 350 strong enough, but I would think swapping to a 400 would be more expensive. The truck will be used mainly for transportation, with a little bit of mild off-roading. I'm not going to even guess how much torque the engine is making, the info above is all I can offer. Any help would be greatly appreciated, thanks.