The basic way is to simply wire in a toggle switch to the OD solenoid. A more "automatic" solution is detailed here:
It's titled for a 3 speed to 518 upgrade, but the same process is what you'd do for your project. Works the same for the 46RH, which is the re-designated 518 with a lockup converter.
You may also find this helpful in moving away from a computer-based system:
And then, ignition solutions are plentiful, from Chrysler's excellent vacuum advance distributor kit used thru the 70's and early 80's, to tunable systems from MSD while using a non-vacuum distributor like the lean-burn unit, to even a ready-to-run HEI conversion unit, as well as others.