Sounds like you may have two doorstops at the moment. I don't think anyone makes manifolds and drives for the 6V71 and 8V71 blowers either, this is a different case style than the normal 6-71 inline blower and the aftermarket offshoots.

You're correct about spinning the 4-53 blower faster, from a theoretical standpoint. The 53 series already was "high" RPM, twisting to a rated 2800 RPM at full load. But the faster you spin it the less efficient it becomes. I'd guess you'd have to drive it 50% over just to get 5 lbs of boost. As the pumps efficiency drops this extra energy converts to heat, which raises the air temperature being supplied to the engine.

While you can continue on with the project I don't think you'll be happy with what you end up with, and it's going to cost a lot more than if you stay with a standard (off-the-shelf) 6-71 configuration.

Sorry to be a stick in the mud.

Regards, Mark