What Mike said, +1, for same reasons. Plus you'd need the STC.
I put Hookers in early C-172: Excellent quality and service, well-proven and engineered STC. At that time (~1992??) they didn't offer inertia reel, but do now, I think. BAS also very good, inertia reel handy for manual flaps, but don't know if STC'd for early C-150. Wag-Aero has STC'd set but I
HATE their buckle hardware: There's a groove on the lap-belt tang that catches the shoulder harness tang about two times out of five

. Imagine you need to exit in a hurry..... I helped install Wag-Aero in a C-140 (structurally about the same as early C-150), and it was straightforward. I wish they'd fix their hardware problem. All of these systems mount to the rear spar "hat-section" carry-through structure, hiding behind the headliner.
In all cases best to use an I.A. with experience in these installations: They don't just bolt on. Check the various websites for installation details. Mike, can you share the citation that TSO is no longer required, specifically on the lap belts? I've been looking into this recently for another installation, and the I.A.'s I work with all say it its.
Crenshaw, is that your C-150 on the Salida airport cam, near Randy's C-172?
Thanks. cubscout