The problem with the TH400/Gear Vendors is that you don't have a lockup converter. The 22% overdrive ratio isn't spectacular either.
If you are using a stock or near stock converter, this wouldn't be too much of an issue, but anything with much stall and the 400/GV setup is going to build heat.
I had a variable pitch converter TH400 and Gear vendors in my '69 Chevelle for a short while. After dealing with "real" automatic overdrives for years and doing conversions it would be low on my list of methods to get OD. And having OD is important in my opinion for any car that will see mileage, whether it be a manual or automatic.
The GV doesn't fit as well as they lead you to believe, and I had driveline angle issues, even with it as high as I could get it without cutting the floorpans. Part of that issue is due to the extra length causing a SHORT driveline, which makes the angles more pronounced.
I would rather rebuild a 700-R4 or 200-4R every year than use a Gear Vendors.
They also downshift fairly harshly.
They may work better behind a manual trans.