so, I guess the question is, what kind of ends are on the belts that make them not bolt up to the factory attachment points?
Are your inertia reel or fixed/adjustable?
I looked them up, most seem to have the standard eyelet to mount to the factory bolts.Is your E a coupe?
Length is an issue, often year/make/model specific.
Reason I asked about attachments is some odd belts have a double hook to mount to an eyebolt in the floor.
https://www.ratsport.com/Files/76817/Img/08/eyebolt.jpg
My MK2 has similar belts and no inertia. PITA to adjust, and for a 1964, they are aftermarket, as they didn't have them originally, and none in the back.
Here is a bit of a tutorial on E belts:
https://etype.chrisvine.com/2015/04/fitting-inertia-seat-belts/
Shotgun position to adjust belts we just move the seat.