I assume you mean the standard bed. Nissan makes a standard bed and a long bed.
On the standard cab and kingcab models the standard bed is 6'1" long however, on the crew cab the standard bed is shorter (don't know the exact length but belive it's about 4') and the long bed is 6'1".
Which model of Frontier do you have?
I believe you need 22" from the back of the cab to the center of the 5th wheel hitch and the 5th wheel hitch mounted over your rear axel (or 2" in front of your rear axel according to the install instructions from Reese Hitches). So, if your bed was only 4' long, I believe you should still be able to pull a 5.0.
Maybe someobody on this forum has done it but I've not seen photos of it. Check with the factory.