for mud racin, you want high hp and wheel speed. ok, 73 k5.
hp60 from a 78-79ford f350 in front with spool and 4.10 gears strongest factory front axle. kingpins, reverse rotation gears.
14bolt rear with spool and 4.54 gears. the lower gears in front will pull the truck out of holes better. alot of comp mud guys do it.
BUILT 400 smallblock. keep weight down, but power up. th400 trans, built to hold the power of the engine.
6inch lift, and CUT FENDERS. keep weight low, but the lift will still allow u to clear 39.5's. we did it, so they will clear.
remove interior to get weight down, cap off, but install at least a 6 point cage. move radiator to the rear of the cage with 2 pusher fans, just to protect from mud. remove all glass so no worry of breaking it. install chicken wire for windsheild.

all items i have seen done with competition rigs that win.