Dave,
Mine are also Happijac's, 4500's on the front (with folding extension brackets) and 4150's on the rear. The inside dimension on mine with the jacks extended is 100 inches. That allows for about 2 inches per side, which seems to be adequate. There is a good view in the mirrors when backing down.
One word of caution once you get everything loaded, if you have a quadcab, the rear door will not clear the camper jack when it is in the travel position. I placed some of those clear rubber bumpers on the jack to prevent door dings .
Dave