Thanks for the responses, I ended up going the safe route and spending hours swapping around the seatbelt bolts, it was horrible but at least it'll be safe now.
For anyone wondering, if you are going to swap pyro seatbelt tensioners on to seats that originally had mechanical tensioners, you have to swap the seatbelt bolt too because they are different for pyro and mechanical. The seatbelt bolt in a mechanical tensioner seat is shorter and doesn't allow the pyro tensioner to sit flat and tight with the seat rail