The 302 was GMCs biggest straight six. Produced in the 50s, replaced in 60 by the V6. 4" bore X 4" stroke. 130 hp, with tons of torque. Became a favorite of the inline hot rodders, but has been replaced by the 292, which has far better parts availability and the block isn't nearly as scarce. The last time I checked the 302 short block was selling for $500 to $800, bare and untested and if you could find one.
If you're wanting to stick with an original look, go with the 261. I believe it was an option in your truck.
