Manufacturer said I will need to use larger than the stock 14" rims, what size have others used...I need backspacing as well.  

It all depends on the year model you have. The mag 500's in the 14 inch size usually work on the 65-66 models, but if you are going to have to get new wheels anyway, I would get 15's  to insure a correct fit.  The 4.75 BS is also suggested.  You don't want to get too wild here though because you will have rubbing issues.