There are plenty of options to retrofit 3 point retractable belts to our cars. Google for 69 mustang retractable seat belts.
Here’s something similar to what I put in my 69. https://www.cjponyparts.com/seat-belt-3-point-retractable-with-push-button-front-pair-coupe-1968-1973-fastback-1965-1973/p/SBR7-V/
Also look at Wescoperformance.com - I think I got mine there.it’s been a while.