Need C20 width measurement for fitting on a trailer (pic to describe)

[jocko]

Does anyone know the below dimension? (i.e. outer track width - am trying to decide which Uhaul trailer it will fit on - there are 2 types, one covers up to 75" and one covers up to 79.5"). Pretty sure I need the larger width trailer, but just want to make sure before I reserve.

I couldn't find the measurement on here or in any of my lit. I'm separated from the truck right now, so can't go out and take a peek.

Thanks gents.