Matt, did you absolutely find TDC mechanically first? And what harmonic balancer are you using? If stock, it's almost a sure bet the outer ring where the timing mark is, has shifted towards retard as age and oil leaks break the bond it once had. Then you'll be trying to set timing to the wrong point on the crank rotation.
NOS balancers are 45+ years old and even new replacement ones sometimes have Windsor or SBF timing marks. Parts store employees that know what a 351-C is are getting rarer by the day, and some will insist that 351-Windsor parts are "equivalent" because their erroneous catalogue says so.