He's probably right about the tranny being from a truck because Dodge made some "Miser" trucks in the 80s that had a slant 6 or 318 that had that 4 speed od trans. My buddy had one that we pulled the slant 6 out of and put a 360 in that was making about 400hp. Fun ride but he was afraid if he power shifted it would grenade the tranny. That tranny is interchangeable with an a or an f body since we compared it to my Duster trans that was still waiting to be installed. Also the shifter boot and bezel in those trucks are the same as an a or f body. Some of the trucks even had an under dash reverse light. (I have a boot/bezel and rev. light from a truck I got in a junkyard a few years ago)
