I had a customers car 71 roadrunner with a 440 that had the same problem. Car came in with a blown head gasket and clacking away. Replaced the head gasket and reset lifter preload as the owner said he tried adjusting them. Rolling the engine by hand I would have loose lifters by the time I got the bank done. You could push the lifters down like slinkies!!! Finally got the to adjust fired the engine and it was noisy for about one minute and settled down. The customer took the car and ran it about three weeks and came back with same clacking and wanted to change the lifters. Changed the lifters and it ran good for about 5 miles and the ducks were back. Just couldn't get the lifters to hold a prime. The engine had about 500 miles on a complete rebuild that another shop did. Only thing I could come up with was the lifter bores had excessive play but the new lifters felt good in the bores. The owner parked the car and is planning to bring it back when he has the funds to open it back up. This one always puzzled me and I looking forward to getting it back to find the answer.