I have done a few unibelt-style 3-point conversions. The older ones used components scapped from junked late-'70s Mopars. They work the best (no inertia reel, actual impact-sensor pendulum) but are more of a hassle to install, hadda have shoemaker do some stitching with nylon thread.
Most recent: In early B-body, I used an aftermarket inertia reel setup from
Seat belt supplier - not as good as factory unibelt system, but a lot better than any pre-unibelt factory 3-point setups, and easy to install. See pix.
Rick
Rick