Picked up a 35 foot camper that I'll be pulling with an 81 W200. Also got a used hitch for a hundred bucks that will need some fabracating. Anyway should the 5th wheel sit right over the back wheels ? Oh the truck is a long box.

 

Assuming it is a standard 8 foot wide trailer. The center of the hitch should be located 52 inches from the cab to allow the trailer to clear the cab when backing up if the trailer is at 45 degrees or more to the truck. This will put the hitch slightly behind the rear axle. If you measure from the center of the pin on the trailer to either front corner of the trailer you can verify that it is less than 52 inches.

http://www.etrailer.com/faq-fifthwheel.aspx

 

Thanks Seal

Good info ya it's a 8 feet wide. Hadn't planned on a camper this big but found it on criegslist for 1800 bucks it'a an 86 Mallerd in great shape so I had to buy it.
I don't think I'll be doing the slant 6 swap now! Be sticking with the 318 4sp. Bought a new 3 core radiator.