Not Ranked
I would suggest that your valve springs are too light, particularly in regard to seat pressure, along with the distributor advance curve and accelerator pump adjustment/shooter size.
The 351c with its relatively large inlet valves is susceptible to having the lifters pump up at low rpm when cold if a backfire occurs. The explosion in the manifold lifts the inlet valve of the seat and the hyd lifter naturally takes up the resulting clearance. The pushrod then bends due to either the valve contacting the piston/ the spring becomes coil bound/ the rocker arm binds on the pedestal/stud/ the pushrod may 'jump' out of its socket in the lifter-rocker------ or a combo of any of these things.. At the same time there is a good chance that the power valve diaphragm will rupture and an extreme over rich fuel mixture will result.
Jac Mac
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