Stock rim width

[madmat]

going to try some 285/65/16 on the 1979 k2500 with 16 inch rims. they are a gm aluminum rim maybe from something newer then a 1979 .

 

[Grit dog]

going to try some 285/65/16 on the 1979 k2500 with 16 inch rims. they are a gm aluminum rim maybe from something newer then a 1979 .

If they’re GM 8 lug aluminum 8x6.5 then they are most likely the PYO wheels. Since you didn’t specify width. Those are 6.5 wide.
285-75s work but are balloons. 285-65s are pushing it imo.

 

[bucket]

I've never even seen 285/65's before.