I'm not sure that there is such a beast out there. Maybe there is a difference in weight when it comes to the cogs of an A-body manual box compared to a B.
Perhaps a difference between a close ratio and long throw box?
I do know that a rack and pinion tends to be the lightest, but that is not going to pass under the relatively stock look.
A carbon fiber manual box would be trick.
The only thing I did was to change the three top cover bolts and mounting bolts on the box to titanium a few years back.