Re-gearing a 300SE for the Highway

[S-Class Guru]

My '91 300SE turns 3500 rpm at 70. This is finally starting to bug me, and it eats up water pumps, alternators, tensioners, etc. I had the idea of regearing it 10 years ago, but never got around to it.

I thought about trading the 3.73 diff gear for a 2.80.
Then modifying the tranny so it starts off in low gear.
Modifying the speedo would be the last part.
I really think the engine could pull the car down the road at 2700 rather
than 3500, with acceleration just a downshift away if required.
I live in Texas, so I don't need the "Granny" low gearing.

So, questions:

1. Anyone ever heard of doing this?
2. Can I buy ring/pinion gears for my diff anywhere, or do I need
to find a complete diff that would bolt into my late 126 chassis?
3. Would I need to find the exact speedo, with the proper internal gearing?
Or can the electronic speedo somehow be adjusted to match different gearing?

thanks,
DG