Modern seat belts in front seat
Don't think those will work in a convertible- no upper anchor point.
With a 'vert, most people convert to modern buckets with integrated shoulder harnesses- one of the more popular swaps are Chrysler Sebring convertible seats; but there are others too.
Of course, if you're trying to retain the original buckets, this is not an option...
Chrysler did offer optional shoulder restraints in A body ragtops, but they were rather awkward.
I'll try to find some links that show how they did it, or maybe someone can chime in with them.