Correct Tires and Rims for '68 Ford P350 Chassis?

[JDxeper]

This unit a, 1968 F-17, currently has 9.5 x 16.5 tires and rims.  most of the tires are old and weather cracked so need replacing.  I was also considering going to new rims 16in, and to 275 /75/16 tires load range  E.   Now I inspected the rear duals and the current rims have almost no clearance between the dually rims and tires.  Looks like bad news on hot summer days.  In my repair manual I can't determine what rims were on the unit originally and what size.  I assume they were some sort of slide or split ring configuration.  I was just going to replace tires but it looks like rims need to be replace also.  Any insight would be of help. 
The rims are not coined like my 78 and the studs are smaller.

[MSN Member]

I have never seen a F model Winnebago with dual rear tires.But it was a chassis that could be ordered by Winnebago.If it is then its a P3500 chassis.

[JDxeper]

Here is the vin plate with the information that I can decode from it.  I have contacted Ford, but the person I had,  knew less than I, which was not much. Will continue to research the tire and rim situation.

http://www.classicwinnebagos.com/forum/index.php?action=mgallery;sa=item;id=1768
I looked in my 68 Ford truck owners manual and it list 8R x 16.5 for the dually p-350 & 3500, me thinks someone switched rims and did not do it correctly. 

[MSN Member]

The 67 and 68 F-17,F-19's came with 7.50 x 16 tires and rims.

[retireddodgeguy2]

My 67 F-17 had 16's bias ply. The Rear end wasn't made for duals. I was able to put my "Budd" rims from a 73 dodge D300 on it but the offset wasn't enough to clear and sit in the well. I think you are better off to return to the 16" single rims with a 8R/9R radial. Mine did well with the 8R single. (I only had the 300 6cyl so it didn't make a difference in power).
Just a thought...
Mike

[JDxeper]

If I can find rims (wheels) that have the correct fitment, will the Dana 60 with a 4.10 ratio work with the duallys OK?  It now has 9.5 x16.5 that are not the correct fitment. unless I go down to a single rear tire.  I am almost convinced that it had 8.0 x 16.5 split ring duallys originally because of the extended rear. Hm? extendabutt
It may have a different rear end than what's on the vin plate also!!
OH well i??  BTW It is also a 6cly 300CID

[Oz]

The extended rear could be a significant point.  Due to the added weight and the capacity of whatever goes in that space, it may exceed the single axle rating and could be the reason why it has a dual axle?