The later wheels will be for 9/16" studs instead of 1/2". (AFAIK) the older 1/2" setups used a flat nut directly on the wheel and the later 9/16" stuff used a clamping plate with conical seats and lugnuts.
http://67-72chevytrucks.com/vboard/s...d.php?t=285970
Assuming you have 7.50-16 bias tires on there now (roughly 660 revolutions per mile), a 225/70r19.5 (around 650 revolutions per mile) will be almost the same. The more archaic 8R-19.5 @ 620 revs per mile will make 50MPH be something like 53MPH.