Have you considered using a 350-C transmission, the one with a lockup clutch in the torque converter? It gives a 10% increase in driveshaft speed when the TCC locks in after going through the normal 3 speed shift pattern with 3rd. gear being direct drive. Getting 10% more driveline speed (or 10% less RPM) without going through an extra set of planetary gears is probably better efficiency-wise than having an OD.
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!