Shoulder belt install

[MCC]

I am looking for some advice on how ot install shoulder belts in a 74 k-10. The problem is that there are a set of factory(?) speakers in the very spot where the upper belt needs to bolt up. I have found the backing hardware that goes inside the rear pillar, but am not sure if there is going to be enough material to hold them in place in case of a collision as a good part of the front of the pillar is removed. If anyone has run into this problem or has any ideas on how to solve it, I would like to hear your ideas. Thanks in advance.