MG Car Repair: Early D overdrive wiring, overdrive units, vacuum switch

Question
As I understand it, the early "D" overdrive has the selector switch, 3-4 switch, and the mysterious "vacuum switch" all in concert to supply power to the OD solenoid. The "relay box" and the vacuum switch are made of "untainaium" and have not been seen in 25 years.  At what level of vacuum does the vacuum switch open?  Can I run power through the dash switch, 3 or 4 transmission switch to the OD solenoid? The down side would be the unit would not drop-out of OD at WOT. Can all of the this stuff be controlled with a microprocessor and a few relays?  Or should I install a chev v6 with a t-5 and be done?

Answer
Hi Robert,
I have seen many cars bypassing the vacuum switch. Also many even bypass the 3/4 trans switch. However, the 3/4 switch is very important because without it you can engage reverse and I have put new overdrives in several Healeys when they backed up while in OD and the one way sprag bearing split the case and in one case the owner forgot he had it in OD and gunned it in OD and the OD exploded the case in many pieces. Even though each of the different types of overdrives used in the British cars have a some different components, they are all designed inside about the same.

I have seen the overdrive out last many of the cars and even seen some of the overdrive units used in rods with large engines and hold up. But I am a firm believer in the 3/4 switch being necessary.
Howard