Thinking of changing trans to a later model 5 speed. What am I in for? I currently have a T-18 with a 351M motor.

 

The biggest issue is that you have a 335 series engine, no 5 speeds were ever put behind them stock. That said they have the same bell housing as a big block and they did put the ZF5 behind the 460. There's a thread in here somewhere about putting a ZF5 behind a 335, IIRC it's by Gary Lewis and titled Dad's Transmission.

Edit
https://www.ford-trucks.com/forums/1...available.html
https://www.ford-trucks.com/forums/1...-zf5-swap.html

 

Save yourself a lot of trouble - both in reading that thread as well as trying to find the parts. Contact Tim Meyer and buy his kit: http://www.tmeyerinc.com