I did a 3 point seatbelt install when I had the roof apart. I didn't take any images at the time but will take some as I assemble it later.
I saw some who mounted the shoulder attachment to the post. I thought that it was too far forward as the seatbelt lifted off my shoulder. I wanted to get the seatbelt about in the middle of the window but didn't want to weld a bar across the window. Above the window seemed too high. I found other users on VV that used the Wesco conversion van seat belts. They allow the attachment point to be higher up as it has a short segment of belt that connects higher up. https://www.wescoperformance.com/conversion-van-seat-belt.html
. If you get these belts get the upper bolt to bolt the shoulder point to the van as well... I did not but realized I needed it later.
There wasn't really a very strong connection point there with much sheet metal. I took some 3/16 angle and cut two short segments and welded them in on each side of the roof support connection. The angle iron fit relatively easily in the gutter just below the roofline. I welded it in along the top of the existing pinch weld in a few spots. Then i took a piece of 1/4" angle and fitted it below the roofline gutter. It kind of looks like ford left something out there anyhow as the sheet metal is confused at that point. I drilled three 3/8 holes through the lower 1/4" angle, through the sheet metal gutter, and up into the 3/16" upper angle. I put three grade 8 bolts in to sandwich it altogether.
You can see the top of the 3/16 angle and the tacks holding them in place. The lower 1/4" angle fitted in and a hole drilled for the seatbelt. i wanted 4 bolts but I couldn't quite fit them. The hard part is getting the wrench on the bolts to tighten it all up. It really just has to happen once however.
For the receiver end of the seatbelt I got the belts that have a hard plastic support on them. With a seat in the van I marked and drilled a hole into the side of the doghouse so the receiver button was pressable above the doghouse. The stock seatbelt mounting used the doghouse on one of the two sides. I welded up that hole and just moved it up. I will make a large reinforcement washer on the back when I do the final install.
For the retractor I saw another poster on VV who fit them into the access cover space behind the driver and passenger. The rollers fit in there tightly but low enough that I could get a bolt through the inner rocker panel behind the rear wheel. I will fabricate a heavy washer for that point too. Then I reshaped the cover to let the belt come out properly. This process was definitely better outlined elsewhere on VV. If you can find it again please post the name so credit can be given.
I read that it is important for some retractors to have the unit in a horizontal fashion. The location of the retractor and the location of the shoulder mount makes that work out. I can't remember if the Wesco retractor type makes that important or not. I might be confusing that with the belts in my other car.
All in all I think that my solution worked very well and the belt is comfortable and easy to use. it also doesn't look too obtrusive or look like it is an aftermarket add on to the untrained eye. it is definitely better than fishing the belts from under the seat each time you want to use them... and will keep my head (a little more) out of the crumple zone in the event of that much feared crash in a vintage vehicle. I will take some photos of the belts installed as it goes together later.