You gotta see this ......
It ran good for a few hundred miles.
OK, so the pushrods are not too long as to cause immediate problems, though they might not be optimum for lifter preload. So the rockers just are not strong enough for the combination of springs and RPM and cam ramps.
BTW, it is not JUST the spring load that does this; it is also the peak RPM's and the cam lift and cam profile too. With the same spring and lift, a steep ramp puts more instantaneous lifting force on the push rod and rocker than a slow ramp. Ditto for higher RPM's. So saying that one similar setup worked is not proof that it will work on every setup. And used stock rockers are going to be more likely to fail.
The 273 adjustable rockers are a decent suggestion. OP, how much lift does you cam have? And what cam? A high lift might push you towards other rockers.
The bits of rocker are probably in chunks and down in the heads, or have washed down the drain hole in the back of the head and into the pan. I would expect most or all of them to be large enough for the pickup screen to block (but can't guarantee that). I'd pull the pan and see what you find.
And FWIW.... yes, modern tech support in large companies is not so hot.... who knows what experience and knowledge the person at the other end has? It is not like calling a place like LeRoy Engines and speaking with the owners in a family who have flowed heads for decades.... I have called Edelbrock tech support, and the person at the other end did not impress me as being any particular sort of motor-savvy individual. I do sympathize that they get all sorts of questions from customers with all sorts of knowledge levels, and they cannot analyze an individual's setup like we can here.