from 37 through 70 chevy 1/2ton trucks had 6 lug wheels. from sometime in the mid 50's they were all 15". in 71 chevy trucks got front disc breaks and 15" 5 lug wheels unless they were 4wd then they were still 6 lug and 15" wheels.

3/4 and 1 ton had 8 lug wheels and they were 16" 17" 17.5"

now some time in the late 80" 3/4 ton trucks went from 8 lug to 6 lug wheels useing the old bolt pattern but 16".

so that is how you got 16" 6 lug wheels on your early truck. it does make it easier to get tires.

as far as the 5 lug spair it is on there to pass inspection