3900's with plugs and socket at the front of the radio are all marked, +8v, RB (roger beep), +10k etc. where all the pins connect to on the board right at the side of the sockets.
So if your radio has not been wired for the function you required just look at a schematic for the 3900 that has those functions wired to switches and copy them.
The +10kc wire goes to a switch and to +8v to move the frequency up by 1 channel (10KC's) so you can get on the Alpha channels but don't forget to switch it off when on the normal channels.
The same applies with the beep, if the beep is wired on and you want to turn it off, you only need to remove L9 (I think it is) at the front right side of the radio and put it through a switch to make it switchable.
How far is it.
Twice it's length from halfway.