SM420 is a truck 4 speed. First gear is called a granny gear. This is probably not what you want for this conversion. You could use 51-54 passenger car front hubs. Then you would have the same bolt pattern all around. As far as drilling the pass car axles for 6 lug, there may not be enough surface area to match up with the bolt pattern. I had my stock 56 truck axles redrilled to match the 5 lug disc brake conversion and it works fine. You probably don't need an OD with a 336 rear end
Fred
