transmission hits mostly the bell housing, can raise engine high enough to get correct mounts in. I have the steel parts from the factory 273 in it now with 69 isolater in place, however there is no way an oil pan will fit the engine sits to low.
Have you flattened the seam from the firewall to the floor pan. If you haven't I believe this will cause your problem. I used 63 -66 Schumacher mounts and with the seem flattened you should have clearance. I say should as it worked for me but I know every car is a bit different.
Can you get your camera in there to take a pic of the interference?
