The classes (M1, M2 and M3) and test loads for cars, minibuses and buses etc are all different. I've not done the tests for a few years, but seem to remember M1 was simulating 20g deceleration and M2 was 10g (would have to read the regs to remind myself). e.g. for M1, if the belts were in any way attached to the seat, then the test loads simulated heavy passenger x 20g plus seat weight x 20g (i.e. very big loads).
What kind of seat is it? I presume something like a bolt in / folding aftermarket one, camper, rock-roll bed etc? I'd think long and hard about the quality / effectiveness of a 3 point conversion and how it works as a system with the seat (there are some rock-roll beds that I think would be horrific in a crash). Rather than new belts, it might be better putting any money into a whole new seat.
Witnessing the performance of some installations under test, I certainly reconsidered our vehicles after having kids. The rock-roll seat got sold and replaced with some fixed, high back, 3 point belt seats that I knew tested OK (with shortest possible legs and lots of underfloor reinforcement). When eventually we got a new van, it was a factory built Combi (not a conversion) in the hope it had also done full crash tests (with dummies etc) rather than just the seat and anchorage pull tests described above. I guess good conversions are OK, provided they have been tested and then actually installed with all the features that were used to pass the test (some converters leave out reinforcement based on their "experience"......)