What is the purpose of this vehicle?
Seems a TBI 350 would be 4x the power it had when new, 700r4 would keep the RPM down as well. . Cheap and plentiful, plenty of enough for short trips, Driving 5 hours in that looks to be a painful experience.
Cummins motors are expensive and transmissions are big and heavy, seems like a cool but poor choice for this if its just a short trip thing.