Hi Dave!
I agree with Mike's comment regarding the trucks as they came originally. D200 series, 3/4 ton,probably a Dana rear axle. A 440 was available in '74-'78 .
However, a Hemi-powered shop/crew truck would certainly be cool and unique!
Having messed around with the Red Express kit, I have some advice. Explore your oil pan options before choosing one. I had trouble doing an engine swap due to the oil pan. The pan hit the front crossmember and made the engine sit wrong. I'd say some trial-and-error is in order.