Will a Tremec TKX fit the low-hump floor of an '81 C10, and is a mid-shift conversion needed when the powertrain moves forward?

We haven't verified TKX fitment in an '81 C10 floor specifically. What we can say:

• The TKX is marketed as the slimmest 5-speed for tunnel clearance, and Speedway says 5-speed swaps like it usually don't need tunnel modifications (unlike a T56).
• It has three shifter locations, and vehicle-specific shifter options exist to keep the factory shifter opening. That's the lever to pull if the powertrain position moves the stock shifter spot.

Mock up the engine position first, then pick the shifter location that lands in your floor opening. Measure the hump before buying.