[bigblockv6]

The difference here is the 1000 series was powered by GMC's own 305 or 351(1966 first year) V6 engines. I1000 series were introduced in 1964 and came with 230 inline six from Chevrolet and later 250 as a step down option, more as a base truck with a more economical engine. Back in that era GMC trucks though sharing the same basic body design were quite a bit different mechanically over the Chevrolet trucks specially when it came to rear suspension & axle assembly, the use of their own V6, different dash and instrument cluster design which required a different wire harness as well. A GMC would be called a Fenderside rather than Stepside, and a Wideside rather than Fleetside.