1970 w/o factory ac heater box re-install struggle

Just felt like this was the right place to air my frustration. I've finally found and bought a brand new heater box assembly, seal kit, and heater core for my 70 coupe since the old box had a gigantic crack through the middle of it. I have it disassembled to the point where I can get the front half of the box into position, but as soon as i add the seal to the intake... everything fails. I'm having trouble compressing the foam ring seal enough to where i can get the heater box studs through the firewall. I'll try again tomorrow when i have more help and some better light. if anyone has done this lately and has some tips, I'd love to hear them.

I got it in, had to get the closest stud to the steering column in first, prevent it from falling out with a nut, then lifted the passenger side of the box using my feet as both leverage and pushing power whilst I had a helper tell me which direction the fan studs had to move, once I had 2 studs through the firewall I had my helper add another nut, then I just had to get the last 2 studs lifted up enough to pass through the firewall. Not an easy installation, but not impossible either. I'm going to give this a 7/10 difficulty rating.