If you internally balance that 340 (with either a forged crank or MalloryMetal) you can use the 273 balancer along with all the other 273 parts (brackets, pulleys, timing cover, water pump, etc) to convert the 340 over to fit your car without having to reinvent the wheel. That would help with the torque converter too. Just get the crank bushing reducer for your converter button. Your aftermarket A/C should fit fine depending on what you use for an intake/carb(s) setup. Same with hood clearance. You'd want to rebuild the 904 to beef it up a bit to handle the extra horsepower, along with a better rear axle assembly and better than stock brakes. Engine mounts and exhaust have already been mentioned.
