hey gismow, I don't know what Dwayne used for getting the distance but what I did was set another set of wheels in between them like it was a 3 tandem axle (with normal spacing/gap). when I remove the middle tires... well wahla there ya go... here my redone spread axle set up. hope it helps ya.
ALSO ILL GO OUT AND TAKE A MEASUREMENT PIC OF IT FOR YOU SO YOU CAN SEE IT.
Its stormin like **** here north of tampa fl right now so give me a minute to get out to my shop. lol