Inverted mount will not work - no clearance for muffler. I don't see how being upside down would change the firewall to thrust washer measurement. I've got it mounted at 90 degrees with a pitts mufller out the bottom. I'm using a 2 peice mount with the backplate of the motor about 1/8 inch from the firewall. This leaves the prop 1/4 inch from the front of the cowl. I'm beginning to think they put the wrong cowl in this kit. By the way it was 62 degrees here yesterday. (Let me say this one more time - no more VMAR). Way too many issues with this thing.