I don't know to which level you want to take this car. If its a true L-78 then it deserves the best. But used seat belts can be found and restored. You just need to search for them.
http://www.ssnake-oyl.com/mm5/merchant.mvc And have deep pockets.
Classic has the Arms.
https://www.classicindustries.com/sh...rts/?q=9738920 Keep an eye out on E-Bay for NOS arms, they pop up once in a while. And here, on this site as well. Get on the FB Nova sites and chevy parts sites. Be vigilant for scammers. They truly believe in the saying "There's a sucker born every minute"
https://www.stevesnovasite.com/threa...-armes.687405/ 90 degree Pin....Answered earlier.