Which generation easiest to swap?

[72gmcdiaz]

Which generation (47-54 55-59 60-66 67-72) of truck is easiest to swap in a 12v cummins without need to cut firewall?

in other words, which of these trucks have more engine bay space?
Ive seen many threads but some have cut firewalls some don't.

Im thinking of getting a 1 ton truck of any generation of these to swap in a 6bt nv4500 and use it as a tow rig