Engine turns free only without plugs...

[BradH71]

New guy here. I've recently acquired my great grandfather's shop truck. a '68 short box stepside 250 I6 with a 3 speed Saginaw. It was last registered for the road in '90 and "the truck ran great" when parked in my family's barn...23 years ago... Anyways, I got it towed to my garage and changed oil, plugs, filters, emptied and cleaned the fuel tank and put fresh fuel in. While the spark plugs are out I can spin the motor easily and the starter will turn it with no issues. The problem surfaces when I put the plugs back in. The inline 6 will turn to a point and then it not go further, not by the starter, not by hand. I've checked rocker movement with the plugs out and they seem to be in normal operation. I've since then ran out of ideas and my online search fell short with very little to no information. Help?