I couldn't get replacement belts for an 83 Camaro, so I fitted competition harnesses. They were about $60 each side.
It needed a simple plate made up to take the top connection, the plate mounts to the existing holes and landing where the belt assemble fits.
The rear mount is slightly offset for a full harness but not noticably so when in use because the mounting for the Camaro is far enough back and not that far off center anyway.
A hole in the floor was needed to take the strap that comes up between the legs.
They are much safer in a crash, each strap is twice as wide as an ordinary belt.
Best of all, they totally freak out anyone at the lights who is feeling like a race. You never need to bother, because the thick belts over each shoulder make it clear you'd smoke em if you could only be bothered with small fry.