Re: 1979-175HP Johnson does go into reverse.
my first step would be to disconnect shift cable at the motor, and shift the motor manually, and see if it will go into reverse. if it does, then either adjust the cable, if it will not adjust, replace cable. if it will not shift, then the next would be to check the shift rod height, some may have turned the shift rod the last time the lower unit was off, throwing the linkage off. if that is not it, then it is a problem inside the lower unit.