It’s a 1977is it both the upper mount and the retractor?
if it is just the upper mount you can use the lower bolt on the rollbar clamp, that is how the 79+ are setup.
for the retractor, a small plate can be welded to the lower part of the rollbar, again similar to the 79+
CCOT has replacement belts with the bracket like @GA Architect pictures. That is what I have, as I didnt really trust the 40 year old belts.
Seat Belts FJ40 - Replace existing damaged or worn Belts perfectly... Set of 4, includes two shoulder harness belts and two lap belts for rear jump seats (all new hardware included). Replace worn belt (using factory fittings)... Complete! Seat Belts Fit '73 to '84 however, belts can be made to...www.coolcruisers.com