Its likely that the outer ring on the damper has slipped. Very common upon old balancers of any make. The rubber gets old, and can't hang onto the outer ring anymore. You'd have to do some research to see if all 250s had their timing mark in the same place, or if the damper could be off of another engine, like a 230 or 292. Is the timing tab integral to timing cover, or is it a bolt-on? On some engine families, its possible to install the timing tab in the wrong location.
If your pulleys don't line up, the problem could be different balancer, wrong accy pulleys, or wrong accy brackets. Is your engine original to your truck?
chip
