I just rebuilt my 351m and there is a "clacking sound" thought it was the lifters so replaced them and it did not change. A mechanic I know said it sounds like wrist-pins, but it come and goes. could it be the wrist-pins? or could it possibly be the valves? At idle it comes and goes also but there is an ocasional miss and when it does that it sounds like one or two valves are floating. The heads are off of a 1976 400, they were rebuilt around 8 years ago but the truck was never used so there was no miles on them until I put them in my truck, along with the 400 they came off of, the 400 is no longer with us, it decided to have "kittens" on night while at idle.

Leonard Bode

 

It rather sound as if your push rods are too long. Since the valve train is not adjustable milling the heads and the decking the block can use up the lifter preload. If this is the case you will burn valves in a very short amount of time.

 

No way you'd have that many wrist pins bad at the same time. I agree that it's valve train noise, and that you should check it out ASAP.