Just now I'm dealing with an OD switch upgrade. Got a 240 OD relay off ebay, which will allow me to utilize a "momentary switch" (aka push button or turn signal). Like yours my H/L are on the turn signal and I don't like the toggle option too much so I'm going with a push button on the shifter. I wound up having to epoxy a nylon bushing into the knob and then cut threads into that so the knob would fit onto the 140 shifter (which came with the B20).
I ruined the 240 knob tring to do that so I pressed into service a wooden knob, drilled it out and fitted it with the 240 shifter knob button and did cosmetic work with body filler and paint after reinforcing the botton in the knob with epoxy. A lot of work for an OD switch, but I really want the switch off the dash and on the shifter knob.