I have a 40' 5er,but my rear stabs are only about a foot in from the rear.From the rear tires to back of trailer is 12'of overhang,and 6' of overhang in the front for the bedroom.
35' pad would work,BUT would I want to fight it onto the pad? Don't forget you need room for pickup to get a straight shot onto the pad.
Try mixing some dry cement in with the crusher run and water it down,it will pack it tighter.
And put it in the rules no digging out the stone pad to level.
The pads I did workcamping,about every other month we had to scrape down the middle crown to the sides,the next year that we stopped there the pads were pretty much level and set.
I know it costs money,but usually cheaper to do it right the first time.
2011 Toyota Tundra DC Long Bed
2001 Harley Dyna Lowrider