I would grab the 4.5 vs the 3.9 Cummins as a starting point. Just my opinion. The Dodge FH is unique to the Dodge 6B and not shared with anything else, however the 2002 and earlier Dodge ISB FH will bolt to the back of the T2 Cons / Gen and ISB02 and older 4Bs (Frong gear train)

Grigg, regarding boxing frames, yes, you will need to know how to properly box in a frame and gusset the cross-members to prevent fatigue cracking. To me it is a simple structural engineering exercise

The problems we face today can not be addressed at the same level of intelligence we were at when we created them - Albert Einstein Or with the same level of $ - Me