I dropped a 350 into my 67 GMC replacing the 250 inline 6. I am still running the Saginaw 3 speed and factory 3:73 peg leg rear end without a single problem. Is this the same kind of tranny you are running?? The Saginaw 3 speed is darn near bullet proof. I have put mine to the test over and over without any hang ups. My HP rating went from 140 with the inline to just over 300 HP with this 350.