I'm the PO that put a Spitfire J-type overdrive into my Midget 1500. But that probably doesn't count here.
The worst PO hacks I've seen are on the Ford F250 I bought last spring. The PO tried to install his own electric trailer brake controller, using whatever wire he had lying around, including lamp cord pulled in half. He hooked his signal wire to the turn signal wire instead of the brake wire, then probably wondered why his trailer brakes cycled on and off when the turn signal flashed. So he "solved" that problem by clipping the one essential wire in the turn signal switch thus disabling turn signals, brake lights, AND his trailer brakes. Duh! There were many equally thoughtful repairs like that on the truck. I'm just about done repairing all the damage. I won't catalog them all here.
My Spitfire's PO put in a modern fuse box with blade fuses, but wired it all with black wire. That's about the only negligent repair on the car, with the rest of the repairs being rather thoughtful, actually, like the SU fuel pump in the trunk.
Scott