no! 74 is hydro mechanical, 78 is straight mechanical shift. yes if you completely change all the linkage on to the recieving motor, sometimes this requires shade tree mechanics.
TD, he said it was a 77 not a 78 and I believe that that the 73 - 77 are interchangable. Of course I have been wrong before so you might want to wait for someone else to verify it.
Did you mean 1978 because what I have is a 1977 85 hp? The only thing I have seen on internet is on boats.net and BRP listing which what I thing is showing that 1974 thru 1977 are interchangeable. 1978 correctly is not showing on this list.