You have it backwards, the 235 and 255 numbers are the width, section width to be precise. The second number in the size designation, you mention 45 and 50, is the percentage of the section width that the sidewall hight is.
What kind of look are you going for? In the widths and aspect ratios you are talking, you're going to have to go to a prformance tire that will be expensive and will not perform well with the stock suspension. they also won't hold up well to the weight of a truck.
Having said that, I would suggest not going much wider than a 9-9 1/2" tire, any wider will rub on stock fenders.
Also, what diameter tire are you thinking? that will determine what aspect ratio you'll need.
