I am building a 390 and dont want to kill the 4spd. Anyone know what is a safe limit for a NP435 driven reasonably.

 

They were used in MD trucks with larger input shafts, so they should take a pickup with a warmed up 390 OK. No hot rodding and speed shifting....

IMO if you are looking to hot rod or drag race, find a C6--much easier on the rest of the drivetrain.

But for truck and towing, and the NP should be OK.

 

Thanks for info, wife wants the 4spd to stay.

 

The np435 won't be the weak point in the drivetrain, as long as it isn't wore out completely they can handle just about as much power as a 390 can make.

A buddy of mine has put at least 40,000 miles on his with very healthy 460, most of that towing an 8,000 lb dump trailer. You'll be fine.

Sam