My plastic cover was held by two nuts on studs in bridges and 1/4" bolts in front and back, I use a flex 1/4" extension to get the rear bolt, I had to tap the side bolt holes. I would check the trans mount, seems the engine/trans has sagged in the rear causing the cover to hit the firewall. If your car is an a/t the parts books will not show a trans mount, just use a straight drive mount and remove the syuds welded in it. Been there and done that.
Eddie.
PS: I too, used the Collis Bros' cover.