For a lighter (but still strong) 6 speed that should bolt up look for the ZF S6-650 6 speed manual from a '01 or newer HD pick-up. The Chevy's never got the NV5600.
The ZF was the standard tranny behind the 8.1 and the D-max. The 8.1 should be the same bell pattern as a small block.
I'm hoping to put one behind my 6.2 diesel. The closer ratios and overdrive would be a huge improvement.
Rene