65 Valiant swap 273 to a 360

There should be no need to "massage" the firewall for clearance - the 360 is the same outside dimensionally as the 273. Now the motor home transmission, that's another story. For starters I bet it's a 727 which means the pinch weld most likely will need to be flattened. The transmission might have the emergency brake on the tail shaft. A pic of the donor transmission would certainly help.