Seat belts were not an option for a 1960 TR3 and while the factory did issue a bulletin describing how install the two and three point Britax belts, I really wanted to use modern belts. While I can't say that I would be safer with the seat belts, I knew that I would be riding alone if I did not fit them as there is no way my wife would be joining me without them. In addition, my experience with the seat belts in the TR6 was that the belts in the TR3 had better spool smoothly and not be uncomfortable to use on long trips or I would be hearing about it.
I chose a set of Beams seat belts supplied by TRF because they looked good and had a nice gray/blue belt fabric option.
The first task was to figure out where to put the swivel on top of the wheel arch. Since my main advisor Stuart was 3000 miles away we played a game I called Seatbelt Battleships until we agreed on the spot to drill. I recall that was square C8.
I made up some beefy plates, two shaped for the curve in the wheel arch and welded on the appropriate nuts for the seat belt bolts.
I drilled the holes in the tub at the designated locations and welded the plates to the tub.
Here's how that looked from inside the tub.
For the mount at the bottom of the B post Stuart suggested making up a bracket with a nut welded to it and then turn up the MIG welder and weld the bracket in the correct location and get good penetration through the vertical section of the floor and into the inner sill.
With the mounts in place, time to test them.
If I were doing this again I would raise the spool up about a quarter of an inch because I did not factor in the thickness of the carpet and the padding. Luckily I got away with it because of the design of the spool.
The belts that I ordered came with a stalk that was around 10 inches long and unfortunately that put the buckle right where your elbow wants to be.
After consulting with Albert Runyan at The Roadster Factory he was able to get me some shorter stalks that now put the buckle just below your hip.
It took a while to get everything I wanted but I now have seat belts that work and are comfortable to use.