I have a 1977 440, freshened up with new rings, rod bearings, main bearings, cam bearings, cam, lifters, timing chain, etc...
It has roughly 3-4000 miles on it now, cold oil pressure with 10w30 bit almost 80, runs down the road warm at 60-70,
Now all of a sudden it has developed a knock under load over 2000 rpm, oil pressure never changes and stays good, checked torque converter bolts, flex plate for crack, exhaust, everything is in order, it will not make any noise in park of neutral up to 4000 rpm, any help of advice greatly appreciated.

What happens if you get up to speed on the road, pop it into neutral and kill the engine?

I'll try that and see what happens, I'll post later

Thanks

Got up to about 60, put in neutral and cut it off and everything just went quiet

Header gasket

Had this problem with a ford one time, spark plug was louse, never happened again.

Headers don't leak at all, new gaskets on heads and collectors, all plugs ok, I just don't understand why it doesn't do it sitting still while revving the engine

can you either power brake it or run it up on stands to see if you can get it to recreate?

Is it spark knock? You mention it needs a load on it to happen.

Have you looked at the plug wires? Maybe a bum set is letting 5 and 7 cross.

Haven't had a chance to mess with it for a couple days but it has a new set of 8.5mm wires on it

Check your firing order real close easy to mess them up especially 5 7.
Check the distributor cap and dizzy itself too.

Firing order is correct, is starts and runs as it should, put it on the lift and ran it up to highway speed with a buddy underneath checking for noises and nothing with wheels in the air so it has to be something not engine or trans related I'm thinking