Is the harmonic balancer true and not wobbly? Chances are the outer ring has slipped, this will through your timing way off.
To check it, pull the #1 plug out and turn the engine till the piston is at the top, then check the the timing marks. The line should be on zero. You may have to do it a few times too get a feel for it, use a small wood dowel rod and hold it on the piston to feel when it stops at the top then starts back down.
A well used 250 should turn over by hand pretty easy with a little muscle.
Joe
