Just my $0.02, but I think the decision you need to make first is if you want the 8-track or not.
There are some "new" radios out there that look pretty stock (Antique Automobile Radio brand is one example) - these will have Bluetooth, aux, etc., but none of the manufacturers are producing new radios with 8-tracks to my knowledge.
My build sheet shows UM2 which is AM/8-track and FM, and I really wanted to stick with that set-up, so I went with the rebuilt OEM option. Problem is, in my experience, there are a lot more original UM1's out there vs UM2's, and the respective costs track with the rarity. So I ended up buying a much cheaper UM1 off of eBay as well as the main and 8-track door lenses (to show the "FM" and frequencies) and sent all that stuff to Barry's 8-track repair in AZ. He also added Bluetooth, aux, etc. Before and after photos below.
-Bill