how to mount a v belt pulley to a rear wheel

Any commercially produced pulley you buy, even a small one is going to be heavy. Die cast even heavier. You have to buy or make a Whizzer type rear sheave. 6 inches isn't going to be able to get you rolling, a minimum of 16" is what you need. I saw a picture of a bike that the guy just welded another 26" rim to the side of the one already on the bike, and ran his belt on that.
The problem with all of these options is proper centering and clearance. Whizzers use a narrow sheave that holds the belt close to the rim. Early board track racers had modifies frames to accommodate the protrusion of the pulley. There is an interesting video on Youtube showing how a guy used a 20" rim bracketed to the wheel with pop rivets. He used 10-12 brackets if I recall. The guy built a jig to hold the pulley perfectly centered while he drilled the holes for the rivets. The great thing about belts is they are more forgiving than chains, so a little wobble isn't the end of the world.