If you can fit a 1717! that would be awesome... i was always under the impression, the bigger the can the more efficient the motor... but everyone says how the 1717 is a bit of a power hungry monster...???
whoops.. i did forget that your truck is a lightweigh machine!!! ;)
But definitely start with 18/44/39, monitor temps and go from there!
what wheel and tyre combo were you thinking of using???