I'm typing off a pretty good drunk from WIld Turkey so bear with me. I
remember that belt system, My 73 Impala had it as well.
The one thing I can think of is to check out the seat belt set up in the
last year the EL Camino was made. You might be able to use it as I would
thing it would be close to the correct legnth of belt you would need
for the type of vehicle your putting it in.
You would no doubt have to do some major modifications in the side wall
of the cab. The way the belt would be anchored in your 73 - compared to
how the belts were anchored ( say in an 88 EL C) are two different
Its probably going to be a big project. The buckle portion on the newer
EL C's may be fastened directly to the seat frame insted of anchpred to
the floor, like your 73.
A body shop might also be able to advise you.