Canadians used K20 and K30, not to familiar with the time frame, but did use GM trucks earlier then the US.
As for tires and backspacing, a no lift, 33" may fit, 32" definitely. A thinner rim and 235/85/16s will look good, with a 4" backspace. As long as there is less then 4" backspace I think any other width of tire can also be accommodated.
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