i have a 66 plymouth valiant with a 7.25 rear end, is there a 8.25 rearend from another vehicle that will bolt into my car without modifications, if not do they make a posi unit for a 7.25?. thanks  

yes they do make a posi for the 7.25 i have one in one of my dakotas the 8.25 rear out of a 88-90 dakota with 5 lug would swap in but is 1/2 inch longer otherwise im sure there are others that will fit