Danny,
After going through this thread, I have nothing to add regarding the wiring or the speedometer cable, except that tire size also enters into the equation when we talk about the effect of the OD on speedometer accuracy. My advice would be to go ahead and get the whole thing rigged up and then measure the deviation. I have managed to get my speedometer close to pretty accurate by fiddling with the tension on the spring that provides resistance to the magnetically induced roll action of the speedometer barrel. But that's a really delicate trial and error procedure, and unless you have a bunch of spare speedometers lying around to replace the ones you screw up, I'd recommend just gluing a correction strip below the speedometer "arrow".

I realize that this matter of being permanently in OD in fourth unless deliberately switched out (Option A) vs the OD switching off permanently automatically every time you down shift (Option B) is a matter of personal preference and that there is no point in arguing pro or con once the operator has decided. So, what I have to say on the subject isn't really directed at you or your friend, but rather at others who may read this in the future while they are still trying to make up their minds as to which way to go. I should add that, if you have B20 rather than a B18 and perhaps somewhat less than normal diameter tires on the car, i.e., optimum acceleration, my comments may be slightly less pertinent.

I have the stalk switch in my PV and found that, when I still had the B18 in there and the OD wired for Option A, particularly in tight passing situations when I needed to down-shift into third for a quick pass, it was extremely annoying to find myself back in OD when I shifted back into fourth and actually losing acceleration when what I really wanted was to keep on accelerating to put some distance between myself and the vehicle I had just passed, or was still in the process of passing, before moving back into his lane ahead of him. That was my main reason for rewiring my OD for Option B. If I were to go with Option A with a B18, I would prefer to have a large, LIGHTED toggle switch on the dash that would make it as obvious as possible whether I was in OD or not when in fourth, rather than the turn signal stalk trigger and an unobtrusive warning light that may easily be overlooked by one who is concentrating on passing an eighteen wheeler throwing slush. If I were to go with the stalk trigger, I would put a really obnoxiously large warning light on my dash and maybe add a momentary buzzer. ;-)

Bob S.
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"62 PV544 (B20, M41), "71 142E, "93 240 Classic Wagon.