I thought the SM-420 didn't start until around 1948. If you are refering to your '46, I believe it would have the earlier non synchronized 4 speed. I have the non synchro 4 speed in my '40 GMC and have thought about converting to a synchronized sm420. Jim Carter sells a modified later style bellhousing that fits the sm420 and allows the pedals of the earlier trucks to bolt to the bellhousing. I believe this is what you would need to change from the earlier non-synchro 4 speed.
I too would like to hear from someone who has done this conversion.
Steve
