Hank, yeah, that would be the way to go, the beloved Jack Halton (and others of course) pulled that move many moons ago. All you would really have to do is to drill a hole using the OD gasket as a template for the lock-out rod. You could split the gears and do 8 forward gears and a Georgia Overdrive LOL.
Believe it or not the 3 speed is in good shape but I will go through it with new synchros, bearings and small parts. No hurry for
my old girl . By the way Hank, look in the left hand corner of my truck's rear window...That's a Jack to honor Jack Halton.