The MGA With An Attitude
MGA Seat Belts - LAP BELTS, Retrofit -- INT-121

Now if you only want lap belts, an aftermarket installation is much easier than described in the book. You can just buy a standard medium length lap belt kit (74 inches), drill two holes, and bolt it in, as long as you drill those two holes in the right places. Both holes (on each side) will be positioned near the lower corners of the vertical floorboard behind the seat.
seat belt attachment points inside the car
The mounting hole near the tunnel needs to be drilled about 1-1/4 inches from the tunnel and only as high as is necessary to put the belt end of the bracket about 1/2 inch above the floor carpeting. If you look behind this panel there is a steel angle framework supporting the vertical rear floorboard. The hole should be positioned directly adjacent to the vertical steel frame member. When you install the bolt, on the back side you need to install a large diameter steel washer that will overlap the steel rail to provide the structural support to prevent the bolt from pulling through the plywood in event of frontal impact. If you were a little suspicious about the integrity of the plywood, you could tack weld the steel washer to the frame.
Outboard attachment point behind the bulkhead Inboard attachment point behind the bulkhead
The mounting hole near the outside frame rail should be drilled in such a location that the nut on the back side will be positioned inside of the leaf spring front mount bracket and just above the spring eye. This location is a little trickier to spot, so plan on taking a little extra time for measurement layout work before drilling. Hold the large nut in position in back before you decide where the hole should be drilled. There is a substantial plate of steel here, so no washer necessary in the back.

POC. Less than two hours of fiddling if you're handy with measuring and drilling.

Addendum, Nov 18, 2005:
My reference to the 74 inch belt kit in the first paragraph was for a set which I installed in 1986 which had length adjustment on both buckle ends. I much prefer the double adjustment belts, as it allows you to position the buckle in the most comfortable location on your hips. Unfortunately, the double adjusters are harder to find in more recent years. Most modern seat belt sets will have the tongue side shorter and fixed in length. These may be designated as 38x20 length. That shorter end should go on the side near the door. The belt can then lie on the frame sill just inside the door when not in use. If it should fall out of the open door there will be less of it to drag on the ground to get messed up. The smaller tongue end will also do less damage if the door should happen to get slammed on the metal tab.

Addendum, March 17, 2012:
Here's another idea for the inboard anchor point using an eye bolt, offered by Art Pearse in Ontario, Canada:
Inboard attachment point with eye bolt Inboard attachment point with eye bolt

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