Check this picture for what the Gear Vendors replacement extension housing looks like. Now imagine fitting that into the part of the transmission tunnel that is about 1/2" wider than the existing Toploader housing.
Also, it’s $3K, while Tom Hall’s T5 kit costs $2250 or so. Tom will sell you a complete transmission with all extra parts for about $4k, while David Kee Toploaders will sell you a rebuilt Toploader with the Gear Vendors overdrive for $4595:
So, I have my doubts that it’s neither less involved nor cheaper…