Just my opinion here...

Having been into several 700R4 OR Currently called the 4L60E

The 700R is a wimpy gear box.....Even the later renditions are nothing to write home to momma about.

I have opened up several of these that were behind 6.2 diesels....Literally shook to pieces.
The clutches are tiny, the sprags are also tiny.

The planetary gear sets are small....a 5 planet upgrade helps....

The factory sun shell is a POS..."The beast" (After market unit) is much better

The input shaft is pressed into the aluminum clutch housing.
The housings break or the splines rip out.
There is a hardened steel piece the presses over the aluminum hub to help it out.

Several other issues.....
The 700R or 4L60 is OK behind the V6 and the 305 V8.....

The factory did not use the 700R / 4L60 behind the 6.5 for a good reason....The N/A 6.2 could break them....The 4L80E is a good durable box and will hold the 6.5....

The cost of doing the swap, plus all the parts that need to be massaged....Drive shafts and such....

IMHO This swap would be a really bad jump.....And I am not sure you will get much if any gains that will ever pay off before the little gear box that could ...grenades...

When the 700R goes bang....It's very ugly, and usually spreads parts all over the street.
I trashed 3 of the 700R in my K5 Blazer......Then dropped in a TH400 with a nice tight RV coverter.....No more issues

The 4L80 does have a tad more parasitic drag due to it's size....Size ....as far as bigger parts is a good thing.
The TH400 and the 4L80 share some internal parts....Bigger is better..


Ma general likely knew what they were doing....

Just sayin....