Another vote for the NV4500 for manual. Or 4l80e for auto.

How fast is fast? Im not familiar with axles available in these trucks, do you plan to keep the stock axels? There are online calculators available for gear ratio and tire size. How big of a tire are you planning on running?

How bad is the old steel? I would try to rework it myself. But that depends on your time and skill level.