Hello Jerry,
Your thought of using a Hurst floor shifter is a very good idea. Hurst was probably the best quality after market floor shifter when they came out in the '60s. If I didn't have an original shifter, for my "Brownie", I would actually consider using a spare Hurst 4 speed shifter I have out in the barn. One thing to consider, and you probably have, is the amount of leverage needed to shift the "Brownie" as compared to a Muncie or T-10 4 speed. If the Hurst is in good condition and with good bushings you could alter the length of the control arms as needed to get the shift lever throw you desire for the handle. I certainly think you have a very good idea with the Hurst. What American-LaFrance do you have? What engine? Off subject, but I have a couple of Alf's from the teens waiting for some time, after the GMC has some attention!
Alan