MGA seatbelt attachment points

[KBEST@novell.com (Karl Best)]

Where should I attach the add-on seatbelts in my '57 roadster? I've
peeked at a couple other cars and they had the belts bolted through the
plywood floor, which doesn't look very solid to me. It would make sense
to go through the side rail of the frame on the left (which isn't accessible
on the outside) and the somewhere on the transmission tunnel on the
right (which doesn't look very solid either).  Suggestions?

-Karl