This might confuse things even more. I only have 77&3/4 inches from inside of the headboard to the inside of the foot board. That is 5/8's less than you. My foot board is 4 inches tall (maybe a 1/16 less). It almost looks as though you can tuck the top (head) part of the mattress under the headboard. I didn't measure how much the mattress sits above the lower part of the head board. When I ordered the rig I optioned to the "Serta Mattress Upgrade" which is also stated as 60" X 80".
On the mattress you are thinking of getting did you physically measure it (sounds like you did) or are you going off a spec?
Sorry I don't know much more but I can always measure my rig for you. Let me know how things turn out.
2013 Ford F-350 SD Diesel, 4X4, Crew Cab, Lariat
Sabre 2014 32RCTS, Pullrite Superglide 2900 18K