Under/overdrive transmission hook-ups

[429mustang]

I recently looked into an overdrive unit, I'm putting a 429 in my 86 mustang and was planning on using a C-6 trans, but cruising on the highway with a tall gear, big motor, and no overdrive isn't too swift. I checked out Gear Vendors site, their units go for around $2200, they can handle 1,200 hp and can be switched from auto, which acts just like a factory 4 speed overdrive trans, to manual which I guess is supposed to split the gears in half, so when it shifts the rpm drops only half of what it normally would. I'm not sure exactly how it works, but it's supposed to be better than a regular overdrive trans and should bolt right up to whichever trans you get it for, but I don't have $2200 to spend on an extra gear so I'm just gonna take my chances with the 3 speed.