The original 318 Borg-Warner trans should be compatible with the 350. Many people think you can change your prop rotation by making an adjustment on the trans. You can change the rotation of the trans by backing off the 4 bolts on the front of the trans & spinning the pump 180 degrees. This must be done if the arrow at the top doesn't show the same rotation that the flywheel moves. If the arrow points the opposite direction as the flywheel's rotation, the trans won't work. You can not change the prop rotation by just turning over the trans pump; that move is made only to let that trans work with that engine. You will notice that when you turn the spline on the front of your trans in one direction, the other end turns the same way. If you go to a 1.5 to 1, the back will turn the opposite as the front. That may be the way to get the rotation you want. I'm curious why you want left hand rotation? Why would it matter in a single inboard? The marina made new exhaust manifolds out of 4" stainless tubing. The motor mounts are adjustable, your motor will still sit on the stringers. Drill new holes in the new location and bolt the motor down once the couplers on the shaft & trans have been perfectly aligned. Use hardwood shims under mounts (or the proper rubber) to reduce noise & vibration. VERY important: measure the length of the new motor & compare it to the 318. Because the 350's were a few inches longer than my 318's, the marina put 1 to 1 trannys on my new motors instead of replacing my 1.5 to 1. Their reason was that they would have had to cut the bulkheads in front of the motors to make room. I was not happy with the results and last fall I decided to do something about it. I cut the bulkheads & mounted 1.91 to 1 trannys on after removing the small direct drives. Now my motors are just sitting on the stringers waiting for a warm day when I have the time to line them up, bolt them down & reconnect everything.