Danny,
aside from the previous advice, I'll share my own experience with the J-type OD.

You'll need a relay or you risk shorting and buring up your wires. Seriously, I had a dashlight short and there was smoke rolling out from uder my dash in the pre-dawn light. The light behind the temp controls snagged on wiper clutch and "poof!" I have no dash rheostat as a consequence. You can use a 3-prong relay easily (same as the reverse relay).

Frankly if you have an 1800's OD light, you can easily use an 1800's OD switch. Fits stock. Works great (Surprisingly) It works even better if you have a proper signal paddle to put on it).

The relay has nothing to do with the OD dropping out when you downshift - that is the job of the 4th gear lockout switch.

If you skip the 4th gear lockout, your "buddy" will be going through Overdrives like tissue paper and probably won't be your buddy for very long. If mounting the switch is your issue, you can "tap" the M40 gearbox lid to accept the switch. It is worth the effort. However, my 67 wagon's stock M40 already had a hole for the lockout - no idea why.

The right angle drive will probably get you around the need to buy a longer speedo cable. The short cable fits too, but will snap quickly because of the contortions.

What *will* get you is that your gearing won't match your speedo or odometer any longer.