heavy four speed ? if it is ya gotta find the heavy 3 from the same time frame as the heavy and light duty trannies have different tranny to bellhousing mounting patterns . i am in the process { not quite the same as mine is a 53, but give you an idea .} of switching from a light buty t-85 to a heavy t-98 four speed . lots of lil things besides diff bellhousings , to make the switch on mine . had to go from a one peice drive shaft to a two peice , bigger clutch linkage bracket that mounts on the tranny , went from a slip yoke { 3 speed } to the saddle the big trucks use on the output on the trannies .. just some of the lil' nitpicking things that are at present slowing me down and driving me crazy .... oh and did i mention the cross shaft / ujoint i gotta go find . one side a bigger size the other a smaller size ........ give me a lil' bit and i'll be back to edit / add more !!!!!!!!!!!!