Here's a copy of a very good post from RV.NET. This should answer all your questions.
I recently looked into converting from 16 to 19.5 inch also(haven't done it yet), as I need to replace 4 tires on our motorhome due to age. Unfortunately, the 1999 and later 19.5 inch F53 wheels will not bolt on the earlier chassis. There are a couple of ways to go on this. Rickson makes a 10 lug, 7.25 inch bolt circle, 19.5 inch wheel that will fit the hub-centered pre-1999 F53 chassis. They do this by starting with a GMC wheel (from a GMC 3500HD, P-series, motorhome, etc.) with the same lug pattern, offset, and bolt circle as the Ford. They bore the center hole larger from 5.25 inches to 5.46 inches, ending up with a 19.5 inch by 6 inch wide wheel rated for 3000 lbs that will then fit the Ford. A recent price for these wheels was about $175 each, plus shipping. If you can find some good quality used, GMC 3500HD wheels, you can have a machine shop bore the center hole larger and accomplish the same thing. A local shop told me they would do this for $25 per wheel. Should you go this route, make sure the wheels have a thickness of .375 inches as this is the heavier duty wheel (a GMC HD wheel rated at 2780 lbs was also produced, having a thickness of .296 inches). Although the 19.5 wheel/tire combination will likely give a firmer ride, commercial, "all steel" tire availability is much better and brake cooling should also improve, along with a larger tire safety margin.
http://www.ricksontruck.com/
I might add that 235/85R16 and 225/70R19.5 tires, have almost the same diameter. aprox. 32". So clearance and speedometer are not a problem.
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