To put a SM420 in a Deco truck you'll need a 1948 to 1953 truck bellhousing. This has the same frame mounts as the original 1946 unit and the proper mounting for the transmission, what's missing is the pedal mounting bosses. Fortunately, forum member Marvin Ribbich has got you covered. Go to this thread and his contact info is in the first post. The picture links are kaput but I'm sure he can get you all the info you need.
Marvins Post


Steve


1946 1/2 ton