I don't like to leave threads open-ended, so I thought I'd share a follow up.
Dropped the tank today, turns out the float on the sending unit was stuck past full, it had gotten stuck past the limiting tabs you can see below. Though once we unstuck it, it still didn't work even with the missing ground wire reconnected. I suspect there's probably more to it than just being stuck, so that'll go on the shelf for now and get fixed or rebuilt at a later date.
Tested the replacement unit I ordered from RockAuto before dropping it in the tank, and the gauge is reading accurately. That's installed in the tank now. Just need to go to the store tomorrow to pick up some new rubber hoses, as the ones that were on there are a little too short.
I'm not sure how much slack these hoses are meant to have, the TM describes that you should be able to lower the tank 5 or so inches to connect or disconnect them, so I'll make some up that are a couple of inches longer than these. At present they barely reach even when the tank is fully in position, the vent and return hoses pretty much fell off when we lowered the tank down, so they were barely hanging onto the hard lines.
And on the speedometer side of things, the gauge itself is definitely broken. I wanted to check if the cable was binding, so I disconnected both ends, stuck a tape flag to the gauge end, and ran it with a drill from under the truck to see if it got hung up, but it's smooth and consistent. I think the magnets in the gauge are probably shot. I will get that replaced at some point, but my GPS speedometer is serving me quite well so it's not a matter of urgency.
