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727A fits small blocks and the 727-B fits the big blocks, 727-RG bolts to the six-cylinder blocks. The 904 comes in a 904-A, 904-RG, as well for some of the earlier 4-cylinders. Some higher performance 904 units were designated as 998s and 999s in the later 1970s.
Also to keep in mind the 727's are longer than the 904's. For a B body example the 727 is the same length as an 833 4 spd. but the rear mount is farther back on the auto so you will need to modify or get the appropriate cross member if changing over.