I have a 1989 Toyota SR5 that has a 94 vortec 4.3L motor with new carb. ,original 5 speed (W56). Truck starts runs fine , First time on road truck stalled out in 2nd thru 5th, runs great in 1st and reverse.Put truck on stands and runs thru all gears, only stalls under load/on road. I have checked timing,installed new fuel pump with pressure regulator (6.5 psi ) for 4.3L. ,HEI distributor, vac advance working. Summit kit was used for swap. Anybody know what I'm missing ? Transmission bad ?

 

Sounds like a plugged up carb. Been through that with various carbureted engines, and sometimes they'll run great until the choke closes. Try a cheap carb rebuild kit before you go tearing out your transmission. Quite often that will fix your problem.