I'm going to use the standard Brownie shifter that has a second stick mounted in the cab. The linkage is a pair of round steel rods that just hug the bottom of the cab and go back to a pair of shift rails on the auxiliary trans. I learned to drive big rigs on a B-73 Mack with a 262 Cummins engine and a Tri-Plex 5X3 with that setup. It was pretty much standard practice for any truck bigger than a 2-ton or so to have an auxiliary transmission and a second shifter. I've driven and/or worked on hundreds of them.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!