The 6.4 SRT cam is over $100 less than the 5.7 cam. The 5.7 springs are a couple hundred cheaper and seem to work ok and have similar spring pressure so I don't know what the difference is but I would get the 6.4 springs if I could swing em. The 5.7 springs do break occasionally after about 150,000 miles thats why I recommend replacing them while you have it apart.
X2 on the spring, the issue with using the 5.7 spring would be in the higher RPMs over 6200, valve flutter
I have run that cam both ways wit tune and with tune, w/tune slightly better fuel mileage, you can feel it come on strong after 3000/3200 only issue I've had with out the tune was some cold weather start ups, and idling but once it got up to temp it ran just like the stocker cam.
the place that I see the gasket failing the most is the slot where the block feed oil to the head bolt that feeds the rocker gear.
