Do a search on Ebay for "Laycock Overdrive". There are 5 of them listed today at ridiculously low prices, and they're the same unit Gear Vendors uses for their horrendously overpriced divorced overdrive. It's EASY to adapt one to the center carrier position of your big bolt.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!