5".
One thing you'll want to keep in mind when choosing tires is load rating. You'll need to make sure that you pick a tire size and ply rating that will at least carry to weight of the empty truck plus passengers/cargo. All 4 tires should add up to this weight. Too low of a load rating, and , while the ride will be great, you'll experience premature tire failure, especially if the tires aren't inflated properly.
Personally, I would only consider LT rated (min 6 ply) for any older truck, especially one that is still relatively stock. P rated would be alright, but make you'll want to go with one designed for truck use. If you're running a 15" rim, 235/75/15 will give you the most choices, you'll want at least a 15x6.5 or 15x7 rim for that size. If you 16', then a 215/85/16 (E load range) will be very close to the OE size, 235/85/16 (again, E load range) will get you a bit more road speed but still keep the narrow tire look. 245/75/16 will allow for more choices in the 16" rim size.
