How gave people mounted bucket seats in 6n's? Has anyone used the frames that are available on eBay?

Theres a few way you can mount bucket seats,

If you are planning on buying a base to go into your existing seat rails I would strongly recommend buying a reputable brand from somewhere like a Motorsport supplier, I think OMP make them for a lot of cars.

Or if you're competent at fabricating some steel can be welded horizontally across the floor of the car to mount the seat runner to.

Bolt in easy fix is buying ready made mounts...most track cars etc have proper brackets welded in, usually when the cage is done, and to get the seat a bit lower.

People do also modify seat bases from other seats, but I have seen some very questionable ones done, to which when it comes to safety stuff like seats it needs to be done right IMO