Like RedRoad mentioned, Gm did make a 6 lug version of the 14b semi float for the light duty three quarter tons, though the spring perch width is still going to be 42.5" and I do not know if the parts are the same as the 8 lug variant.
http://www.fourwheeler.com/how-to/tr...rs-and-locker/
I though I read somewhere that you couldn't convert a full float to a 6 lug, I think the hubs are too big.