Skip,
There are hub piloted rims and lug piloted rims. The ones on your 20 year old small car are lug piloted (i.e. the lug nuts have a tapered section that aligns the rim to center on the hub). Big trucks are hub piloted. Which is to say that there is a fine fit between the hub and the center of the rim. These lug nuts are flat on the inside and simply hold the rim against the hub but don't align the rim in any way.
My 1958 3800 dually has 19.5" rims. They are hub piloted. The lugs fit snug through the lug holes in the rim. The rim center fits snug to the hub. When torquing the 8 lug nuts down to ~100 ft lbs it makes your rims and hubs a one piece structure capable of with standing severe side loading you might encounter hauling a load around a corner.
Now you say you had 17.5's and are looking to change to 19.5's. The clearance should not be a problem (front to back), but you might have to lift your truck an inch or two to get top to bottom clearance. If you never plan on carrying a load then don't bother.
I am concerned about your large stud holes on the rim and small studs. You likely have 1/2" studs but your rims are made for 9/16 studs (I'm just making an educated guess here). This is NOT a good situation. Once again, carrying no load and driving around the block is probably no problem, but if you carry 2 tons of gravel 500 miles on a windy road... that could spell disaster.
You can either find some spacers and put them on your studs, but a 1/32" thick spacer will not last for long. Or you can have some spacers welded onto your rims. Or you can drill out your hubs and drums for the 9/16" studs. This will require more effort, but is perhaps the best idea. You'll have to get the drilling done at a machine shop. You'll have to remove your hub/drum combo (easy on these full floater axles). Take it to a shop and have them press out the old studs. Then have them drill for 9/16 studs and install them onto the hub / drum. While you have it apart you might consider getting some new wheel bearings and possibly brake shoes.
My $0.02.
