Synchro mentions the top mount should be 550 mm (21.6") above the seat. I believe he was talking about 550 mm above the bottom seat cushion, not 550 mm above the rear of the seat back. Our 2014 Buick Enclave attachment is 24" above the seat cushion and our GMC Yukon attachment is 25" above the seat so Synchro's approx 22" is in the ballpark with both of our moderns. This puts the belt just about even with a normal height shoulder, although some newer cars have adjustable attachment points to accommodate different sized passengers. 550 mm above the seat back would put the attachment point through the roof on most moderns. In my '31 A roadster installation, the shoulder strap that goes over the seat is 21" higher than the cushion and just fine for my 5'6" frame.
Further, several sources like Seat Belt Planet, Retro Belt and others catering to the passenger car business recommend the angle of the belt from the top of the seat back to the rear attachment point be 0* to minus 40 to 45* down from the horizontal. The racing suppliers like Simpson, Schroth and the SFI Foundation recommend the attachment be 0* to minus 20* for competitive driving.
My roadster installation retractor attachment is less than 20* and within all the guidelines. So, it is not only possible to install 3 point retractables in a Model A roadster but the installation works quite well.
There are many who feel seat belts of any kind are useless or unnecessary and I respect their opinions.......but totally disagree. Just my own opinion.
Glen