^^You can also buy nut / threaded bulkhead fittings such as in JIC/ AN and after making a clean hole in a flat spot, you could reach a finger or two through the sender hole to tighten it up.
I silver brazed a tube in mine, but I hesitate to recommend that, explosion hazard. What I did, was to disconnect the tank vent fitting, and extend it upwards with a scrap tube. I drained the tank, and ran CO2 from my MIG setup through the fuel line and tank, and it vented out the cap, which I later installed, then continued to vent out the temporary tube. I lit it several times to see if the escaping fumes were flammable, and after a few minutes, it would no longer light. I ran CO2 in there for several more minutes.
I made a hole sort of above and aside the sender hole, not with a dril, but an awl, then used a tapered sheet metal awl to enlarge the hole, which of course leaves a big lip for more thickness.
Then silver brazed a scrap of tube in. Frankly, I still don't know for sure how safe that was, and have read CLAIMS of not good outcomes.