i have put three belts, two shoulder and one bench in my 51
i did (now that i'm doing a complete rusto,) cut two slots in the seat frame about 2 1/2'' x 1 1/4'' right at the sheet metal right angle bend to fit and make it an easier 'pull' adjustment when seated.
these are centered @ 1/3 and 2/3's along that seat frame back-bottom.

belts from juliano's

http://www.speedprint.com/Deves50/seatbeltassembly.php

Jim & Caroline
The highway is for gamblers, better use your good sense."
Gooday-that's my 1¢ answer due to the lousy economy ~ cause I ain't got - no . mo . doe
Every Shaver | Now Can Snore | Six More Minutes | Than Before ... | Half A Pound for Half a Dollar | Spread On Thin | Above the Collar || BURMA-SHAVE