You seem to have a valve piston contact.
You will need to retime the cam to the engine...try turning the cam either way....or loosen off the valve gaps ...(.EDIT.)
Unless you can gently turn the crank again while feeding in the new chain, the above any be unavoidable.
Duh...Of course....you can pop out the rockers to get clearence to turn the crank/pistons so nothing collides...brain fade here.
I've still got my head in this 603 !
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