slant six 3sp. to 4 sp. conversion

hey...this is my first manual trans a-body. its a 74 duster 6cyl with a 3 sp. manual trans. im going to use this as a daily driver,and would like to change out the 3 sp. for a 4 sp. is there anybody who may know if there were 4sp. 6cyls.? what years? im guessing a small block trans wont work...but dont know for sure. any info yall can pass along would be appreciated!