Gear Vendors Overdrive?

[rockertype]

I really wanted to install a 4L80E in my 67. Unfortunately, no transbrakes are available yet for this transmission. I want something bulletproof & with some type of overdrive. I'm now considering using a "Gear Vendors" unit with my TH400. Does anyone run this combo? It's operation seems a bit confusing ("splitting" gears). Shifting, pushing buttons, ect. Are they easy to use? I really just want an overdrive for cruising but I must also have my transbrake! Can I just shift normally then use the GV's overdrive for cruising?

Any pointers would be great!