4 spd/3 spd interchange?

Looking at a slant six Demon that has the factory 3 speed in the floor so all the pedals and such are in place. My questions to the more knowledgeable is this. If i were to consider, which i am, changing to a 4 speed what problems, if any, will i run into? Will the same bellhousing work? Is the spline configuration the same on the 3 speed as the 4 speed? Pressure plate the same? Shifting fork location the same? Shifter hole in floor the same location? You get my drift so if you know please clue me in as to the magnitude of this swap. Thanks guys
Small Block