Well guys, I chickened out on the diffy swap - again.
I would need to make it start in 1st gear everytime, and reading some old posts about that modification were not too reassuring.

But the copncept seemed sound - gear up 40%, then start off in 1st and actually get 15% lower gearing on take-off. Then get 40% overdrive in 4th.
I compared the proposed gearing, and it almost matches exactly the 1-2-3-4gearing on my little PT Cruiser. That little 2.4L pulls the 3200 lb PT really well, so I thought the 3.0 might ease the 4000lb SE along sufficiently..

I did a search on 1st-gear starting. There were some old references to Bergwerks having an electrical shift-mod method, but I can't find that website anymore.
There was a guy who modified valve bodies for $500, don't know if he is still performing that service.
He shot down the quick/easy method of changing one spring in the valve body - said it would not be satisfactory.

Then there's the question of "would the 1-2 shift be acceptable on a daily basis". It can be pretty jarring at anything more than 1/8 throttle.

Then, "would the 420 diffy fit for sure", and be in good shape out of a used car, and "could I successfully re-calibrate the speedo".
Then, "could the little engine successfully pull the car along without having to downshift to 3rd all the time". Ha, I had an old Eldo with the 4.1L that downshifted when I met a truck!

So, maybe i'll just apply the 150th coat of wax to the Benz, and keep it as the Town Car. after 14 years of good service for us, it probably needs a rest from the long trips anyway.

DG