Brian, I've suffered for 20 years with the exact same issue. I seriously thought about a swap; but upon investigation of a viable 1st gear start modification, I came up a little confused. Some old posts herein said to just swap a spring in the valve body - but followup posts said that trashed the tranny in short order.
One old poster was said to modify the valve body for about $200, but I couldn't find him.
Another post talked about an electrical add-on that tricked the tranny into very quickly shifting down to 1st on startup. That seemed to invite potential rough shifting in my opinion. Then, I observed that when I manually placed the shifter in 1st, the 1-2 shift had a tendency to be pretty rough.

Looking at optional diffs, I concluded that the diff you mentioned would seem to be the correct one, but it might tend to downshift in a high headwind, who knows? Also, it seems there are various configurations of diffs, so I assumed I would get one that didn't fit, when I found the proper ratio.
So, I put it off, and now 20 years later I still have the car; but now it's relegated to in-town use only, so problem solved.

But, if I had it to do over, I would certainly pursue it more dilligently.
The engine has seemed to survived the high revs easily for over 200k miles, but I have gone thru a bunch of accessories like water pumps, fan bearings, etc; that might have lasted longer. And, 18 MPH on the highway is pretty tough these days.

Go for it!

DG