The original tires on the 1978 K-30, per the chevy brochure, were 31x9.50x16.5. That's a 31" tire. The 235-70R16 is a 29" tire. Your speedometer right now with the shorter tires should read 31/29 high, or about 7% faster than you are really going.
That assumes the rear-end ratio was not changed since new. 3.73:1 sounds like the stock ratio.
Not sure which speedo gear is in there now, so hard to tell you which one to switch to.
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