I added seat belts to my 41 Master Deluxe. Bought them at
https://www.seatbeltsplus.com/. Ordered their 2 point lap seat belt and the installation hardware. I bolted the belts to the floor and came up between the upper and lower cushion in the seat. I am not using the original seats so the install may be different for you. Nice part with the new ones is they are clean, color I want and the proper length.
When adding the belts you will need the large diameter washer to keep the bolt from pulling through the floor if there is an accident. The $5 for the kit that they charge is a good price considering everything you need will be there.
On a side note if you do go to a junk yard to get your avoid any that are from cars that look like they have been in a bad accident or belts that have stretched stitching. Last thing you want is a belt that is damaged due to already doing its job ones.