BRIANSKI Brain Not sure with your motor, what damage from the built and have not seen any pictures of it.
The last couple of years there has been a MAJOR problem with
oil for the motors with not having enough
zinc and other materials that make motors run 100K in miles. Machinist all had different ideas on helping make a motor live longer, with solid and hydro lifters. Lighter valve springs was one,
oil additives, removing valve springs for break in to 1,000 miles. The cheap lifters where failing with some of the best builders in the world.
You said you had the car checked out, compression test, leak down test,
oil leaks, coolant pressure test, and oil sample test. These are some of the outside tests on a motor. I don't know of too many sellers that will allow you to get into the motor, pulling valve covers and oil pans.
You say that the motor has some miles on it and a couple of heat cycles. The springs may loose up to 30 pound of closed pressure after breakin. You get what you pay for with this. The other thing is what rpm are you turning with this motor. If it's running a FE bottom end even with ARP bolts in the rods, a 6,000 is a safe limit. Running high rpms that, just shorting the life of the motor. Light spring will stop you from over revs. Valves start to float and you loose power.
If there is wear on the valve train, there could be also other issues.
The motor is a solid lifter block? or you are just running a solid setup? I don't think a 428 came as a solid lifter block, but will check. Any way are you running a windage tray? This cuts down on the spray and splash for the lifters and camshaft in the block. Valve lash, if looser than spec, perwear is going to happen. Oiling system where any modifications done to it? Oil pressure at idle and 2,000 rpms hot? May not be enought to lube the motor properly. What kind of oil was being used?? Running a higher rate valve spring may and could do the same damage again if these questions are not answered and fixed. Run the motor with 1 extra quart of oil because of FE motors having poor returns to the oil pan. Does the motor sit for a week or more before running? If so, you have a metal on metal startup. A preoiler may be a safe way to stop this. I know they are not cheap but going to roller camshafts and lifters are better than solid lifters. Rolling beats rubbing, even dry. The solid roller lifters now have pressurized oiling for the pins and bearings and have extended the life to 20-40K miles. As for rockers, only get ones with bronze bushings or bearings. Heavy duty shafts and most important good set of shaft supports with end plates. If you have any pictures, could you post them. Rick L.