You can find a common 427 Tall Deck engine that came in heavy duty work trucks, the decks are higher so the use of intake spacers or a tall deck intake comes into play, they will also require special headers since they sit higher so you may run into clearence issues. Stuff a stroker crank in them and long rods, and have a monster in cubic inches, and a monster power maker. I do believe several on this board has built one of these with very impressive results. Of course a aggressive cam and good high flow heads will be needed. We used to mud race and alot of factors come into play. when the mud is thick and has some depth to it in spots, we learned quickly run the narrowest tire you can, as there lighter weight helps and they lower the resistance alot by not pushing as much mud as wide tires do, wide tires also tend to "ski" out of the ruts. We also learned don`t gear it too low, as with the transfer cases 2:1 reduction and with low gears in the diffs, the wheel speed will be too low to win unless you shift to second and then you almost have to have a big block to keep the engine from lugging. We ran a 79 GMC high Sierra, with a slightly warmed 400 small block, 3.08 gears, 38 inch buckshot mudders in the narrowest tread we could get them in, 3000 stall converter (note: I`ve never seen a stick shift win in mud racing after 6 years of being involved in it) and a must have 4 wheel posi traction, any tire not pulling is dead weight which is the same as dragging it along. We have many 1st place trophy`s for our efforts, and several 2nd placers as well. The launch, wheel speed, narrow tires, all play a big role at winning in mud racing, Torque is also a very big factor. Good Luck.