1970 c20 engine options

[atkryder]

My father in law recently gave me a 1970 c20 arizona truck. The plan is to fix it up for a tow rig. I will be mostly pulling a 18ft bumper hitch trailer with my rock crawler on it but would like to have the option of a gooseneck to maybe haul a tractor. The question is what motor. It has to be fuel injected and auto tranny.The options i have looked at are:

FI454(almost as much as 6.0)
FI350( underpowered?)
5.3 (maybe underpowered?)
6.0 (expensive but maybe ideal)

What do you guys think?

Seth