I would not purchase the tank. If it's already yours, drain it and either give it to scrap OR condemn & create artwork.
A 40yr old Al tank puts it in the era of using 6351 alloy, which was never officially recalled but has the issue of suffering sustained load cracking. Fancy way of saying it can explode just sitting in storage even under best conditions. Where as most tanks explode during the filling process after a tank has been damaged or compromised.
You can see the link for more info.
PSI-PCI - A short Review of 6351 Alloy Aluminum Cylinders
It requires an additional Eddy Current test during visual inspections to be deemed safe for use. Aluminum tanks built 1990 and after do not use this alloy and don't have a history of suffering from SL cracking. It would be your responsibility, for your safety and others, to ensure that dive shops are giving your tank this additional test and paying the cost associated with it.
Some shops may also outright ban you from bringing the tank in to fill, due to it's notoriety. Their right as a business to do so.
J valves have been on-off current market production by XS Scuba within this decade. Looks like they might have stopped again. Some divers, recreational and commercial, has found niche uses for it. By and large though your un-familiar dive shop may not fill it correctly and give you a tank not filled at all. Really just creating an inconvenience if you're doing your due-diligence function checks before diving.
Older J-valves may or may not have current 3rd party service parts available.
