i dont have my brb in front of me but i'm pretty sure it does say how many firing points on each facing of a bastion.
the emplaced heavy bolts have alway been a loophole because it says something like "4 emplaced heavy bolters, GENERALLY one on each facing". Every one i've seen in a tournament has been the intended one on each facing but i'm guessing chase must of ran into "that guy" who kit bashed his to have all 4 on one side thus prompting this house rule
It doesn't specify firing points, it says "as the model" by which, obviously they mean the GW model. Now it's a little funny, because you actually have a few different options in how to assemble the GW bastion, so it's not exactly fixed which sides get what fire points, but regardless you can only get 3 firepoints on a side, and I think you can only make that happen for one side.
But trying to use another model with significantly different characteristics is silly WAAC nonsense, and it also means you're kinda jerk, I would recommend not playing with those people.
NOVA has a fairly sensible modeling for advantage policy: basically mad conversions are fine, but you're to use the worst of EITHER the original model or your mad conversion. (height, etc). FOr instance if you converted a demon prince to be double height, that's fine, but you'd draw LOS as if you were the original model, but could be shot at as if your current full height.
I thought BG followed something like that. If they don't, they should, it covers most situations.