the stock 351w heads are not slotted in the pushrod valley. so if its a large cam it will cause the pushrod to go against heads and bend the pushrods, second check for coil bind need about 80 to 100 thousandths for clearance.
third make sure your valves are set with pumped up lifters, especially on hyd. lifters, if set non pumped up can be set to tight and when lifters do pump up it will be to much and put you in to coil bind.
my question? are they a Rhodes style lifter, a anti-pump up.