John~Not providing all reverent info voids all guesses
the lifters and cam are new. Hydraulic flat tappet .
Sounding more like oil pressure than short push rods
The lifters, just because there new don't mean there right for you application.
might not be push rods at all then, maybe lifters bleed down too quick or not enough oil to em. 351C Lifters or pushrods should have the small .040 oil hole for oil restriction.
Real Oil Restrictors are for solid cams only.
When the engine heats up I get a valve tap that is pretty loud.
No noise when cold?
Whats the oil pressure like hot?, cold?
What weight oil in her? Might be as simple as switching to 20w50
Did ya put a high volume oil pump in it too?
Any checking of the pump and high-pressure relief spring ( one in pump)?
In 6 mis of warm up maybe all oil has been pushed to top of motor causing your noise and starving your mains, rods, and lifters!
What else are you not telling!?!
would it be smart to use adjustable push rods??
I know nothing about adjustable push rods except they use em on harleys sometimes.