Wow! Sounds like fun.
That'll be a pretty tight fit, but manageable in a 56 I think.
Does it have a turbo?
Is the transmission an OD?
I ask because for the effort involved to install a 6V53, and it's weight you really want the turbo. A non turbo 6V53 has got the same oomph as a 4-53T, which is lighter and a good bit smaller, making it usually the better choice between the two. Obviously if the 6V53 has a turbo and it fits the truck then that's a good choice too.
You'll likely need an OD transmission and or some pretty fast gears (low 3.xx) to make it enjoyable on the highway.
Got any pictures yet?
Grigg
