In the ls world the lifter for the auto cars can not take the spring pressures needed to control the valvetrain of most aftermarket cams. It is necessary to swap in non-DOD (gm's MDS) lifters. The high spring pressures can cause the lifters to colapse.
Check out the specs for a stock cam and the 6.4 cam.
6.4 with mds......
Intake 286 open@36*btdc/closes@250*atdc
exhaust 288 open@278*btdc/closes@10*atdc
46 deg overlap
5.7 with mds
.105" more lift on the intake and .067" more lift on the exhaust with the 6.4 vs 5.7. I'd like to know how much "cushion" there is untill spring bind with the stock cam. Another question is the amount of lift a 5.7 MDS lifter can take and still be functional. .105" is a lot of lift!
Maybe the 6.4 cam can be used with MDS shut off with the stock MDS lifters. But why use an MDS cam then?
Obviously by the different part #'s Dodge felt that to run the 6.4 cam they needed new lifters and springs. Seems to me they re-used as much as they could, since all the other parts I listed have the same part #'s.
The comp VVT cams are designed to retain VVT, not MDS. That is why I mentioned solenoid plugs and standart lifter swaps. If you think you'd be able to use the stock springs with the 274 cam you're crazy!