It takes a ton on persuasion to get my stock STB in so Im not surprised youre having problems with this one. In my opinion, after 100k like my car, the frame is going to settle in a final position in respect to what it was when produced.
Try getting the firewal bolts started. Then the bolts on the side of the strut towers. Then the ones on top of the strut towers. Thats how I have to do mine because if I just set it on, maybe on 5 bolts line up.