With all the parts for the swap at your disposal, I'd say "go for it" if it makes you happy.

 

The final drive ratios are nearly the same, so there is no advantage to the sixth gear.

The fluid driven torque multiplication power of good triple disk billet torque converter might be stronger than a Southbend clutch and an aging knee.

I remember when a 7.3L powerstroke drag racer named Brian Kinney (PsychoStroker online) did the opposite swap, from a ZF6 to a built 4R100, in order to gain more advantage on his sub 12 second 1/4 miles.

 

The zf6 has a low top gear but all the zf6 trucks get better mpg than the 4r100 trucks. I don't race so speed is not a problem. Power mpg gear control for towing is all want.

 

Those are the reasons why I ordered mine with the ZF-6 years ago. I'd say it's paid off.

Don't forget the floor hump. You need a taller hump for the ZF.

 

I have one in the yard.