OK personal opinon here. I'd build the Ford. Seeing We did one for my dad a few years back out of a 1983 f250. Pull that 351m out of there and a 460 will bolt right in with minor gas axe action. have to make room for the oil filter in the cross member. Bell housing will fit just up grade the clutch when you have the 351m out of there. You say 5 speed don't matter your using low range 2nd gear anyway. just put 4.11 gears in it and a locker in the front, we had a dana 61 in the rear with nice 35 spline axles, never broke an axle in 5 years of pulling.
What we did with the twin I beam front was put a 6" lift in the center

and left the outer alone so when the truck pull onto the track it looked knocked knee.
But when the front would lift as it pulled down on the back the tires would level out flat.
Daddy campaign this truck for 5 years before he passed away. It was the best 5 years of fun he had.
Dan