Are the electronic boards the same just different face plate,
Or is it a while different unit?

I believe it would only be a couple of different resistors on most, but once into motronic (3.2) it's a different animal. Most use the correct tach for the engine for the redline.
Tim did an thread a while back on how to put a modern tach guts in your VDO, that's the route I'd probably take.

send it to PAS (Palo Alto Speedo) they will do the conversion - probably cost about $200

the have been doing my instruments for 20 yrs - imho 'way better service and attitude than the SoCal guy

Or use this one

Building a replica 914-6 tach from a 914-4 will be my next how to write up. It will involve replacing the faceplate with a new machined one, and placing a decal over the plate from my new friends at whitegauges. net.
I will then show everyone how to calibrate the cheap 30 dollar tach to work with the new 8k rpm sweep. I'm just waiting for parts to come in, but hopefully soon.

The tach adapt seems like a good option. Or you can follow my thread on how to install a new tach drive that will work with a six. In my opinion, I want both. I want a tach with a modern drive, but to also look like a real 6.