I just posted this today on Team Chevelle, so I copied it for posting here.
I installed a Gear Vendors unit in my 68 hardtop with the 511 and a TH400. I think I paid around $2500, but I Compared this to a built 700R4, which I expected would break every now and again, and opted for the Gear vendors unit. The unit works very well and effectively changes the 4.10 gear to the RPM of a 3.20. At the time I bought mine, I had read an article (National Dragster??) that said they were rated to 2000HP.
Anyway the Gear Vendors unit and all the installation parts are very well designed. And the tech guys are very knowledgeable if you have any problem. I had 2 problems though. The first is that the unit would not work with my crossmember without extensive modification. Like most Chevelles, mine had the "tube type" crossmember. Rather than cut and weld, I simply grabbed another
crossmember that a buddy of mine gave me, it was out of a GTO and it is a "stamped"
crossmember rather than the tube type. These are in alot of 68-72 GTOs, Skylarks and Cutlass. Although the holes did not line up in the Chevelle perfectly, I slotted the crossmember bolt holes on the passenger side (where your downforce is).
The other problem is that the unit would not clear the trans tunnel right where it necks down before expanding to meet up with the front seat mount. However, I do have the original 307 frame mounts On my car and am holding the engine a little higher than it should be, so this likely was a contributing factor. Anyway, GearVendors said that they do have to massage some trans tunnels with an air hammer. I tried this, but needed too much clearance, so I was trying to displace too much metal. I decided to simply cut the tunnel where it necks down. Besides, I had wanted to weld In a piece of metal plate to more securely mount the shifter, which I did, and then I formed 2 sections of sheetmetal to cover the rest and I secured with sheetmetal screws.
As far as using the unit as a gear splitter to gain an extra 3 speeds (1st-1stO/D-2nd-2ndO/D-3rd-3rdO/D), that would work on the street in a towing application, but not at the strip, at least not without a timing computer and an air shifter. I have however shifted into overdrive a couple times at the end of the track. But the unit does take about 1 second to shift. I think the actual spec is 0.8 seconds, but I can't recall where I read that.
Anyway, my Chevelle runs high 11s at the strip on true street tires (Dunlop GT Qualifiers) and the unit has held up fine, so I wouldn't have any concerns about it's ability to take the power.