convert 14 bolt 8 lug to 6

[BigBlocksRule]

8-lug semi-floaters have the bigger brakes, just like a SRW full floater, so you'd need to swap everything (unless you're going with discs) to use the 6-lug axles. There are two different widths of semi-floaters. 4WD is about 3" wider and will give you wheel/tire fitment fits, make sure you get one from a 2WD vehicle.
I haven't seen any semi-floaters from the 80's, only the '88 - '98's.
Cheapest bet is to find a 6-lug semi-floater to start with. I bought one with 3.73's and G80 this weekend for $150.
And to call 'em "light duty" is selling them very short. I had the chance to measure axles on one of these and a full floater at the same time. The thinnest area of the full floater is SMALLER than the thinnest area of the semi-floater, meaning that if they're made of the same material, the semi-floating axle is actually stronger. I've never seen anyone break a semi-floating axle, those honkers are HUGE!