35x12.5R20 will fit on my factory 20's with a leveling kit right? Looking at the Toyo m/t or a/t. Want to make sure they fit first.:

 

Yes, if you have a 4x4.

 

Are there any negative effects of putting a tire on a wheel that is not as wide that the manufacturer recommends the tire be mounted on ??

 

Installing a tire on too narrow of a wheel can cause the center portion of the tread to wear slightly faster than normal, similar to running too much air pressure. However, the OEM 20's are 8.5 inches wide and most tire manufacturers recommend an 8 to 10 inch wide wheel for 12.50 (305 to 315mm) inch wide tires

 

I just noticed that BFG recommemds their 325/65/60 be mounted on a 9 - 12 inch wide but now I see that is a 35x13 (the set up that Nebraska has on his OEM 20's). I guess they key word is "slightly" ! A shop in my area looked at me like I was crazy for thinking outside of manufacturer recommendations. So I will be looking for a new shop to help me out. They we all set to sell me some wheels with some wacked backspacing and offset which would make my truck a perfect mudslinger !! which is not what I am going for !