It looks to me like the eassult heads are procomps and the assult rotating assembly is scat. And its exteranlly balanced. So its really not great for 7k+ rpm redline. h beam rods and forged crank/pistons would be better. It will still work but might not be as good if your going for max power na. But this will easily take 500hp. I would focus on that level of performance.
With those heads and rotating assembly. I would stay with the 12-225-4 magnum. I got a feeling those heads arent going to flow real well.
Pro built powerglide like vinnie said is what you want for straight race car. Th350 is heavier and has more power loss. Its popular because it can be built cheaply and is really much better on the street than a 2 speed power glide.
Is this your first race car. If so i would focus on the rotating assemblies 500hp range. Its going to come apart at 800 hp. Also 500hp will take you down the track very fast. You will have your hands full getting it in a car and going down the track straight. Also at 500 the weak stuff will start breaking and you will start apending your racing budget on saftey equipment.
Sorry not trying to confuse you just want to help you get something together that will last a few race seasons and provide the punch needed to move you down the track. 1/8 and 1/4 mile cars usally are set up a little different. Might want to make a choice on what ypur going to run. Gears are usally different for 1/8 mile.
Did i hear a "mud racing" in there.... Mud drag racing usally requires a pro to build the engine cause you will be bouncing around and engine will hit the rev limiter and fall on its face. So they build the motors to "cam out" and not over rev or invest thousands in electronics.