Magnum questions?

Going to .550 lift on a stock Magnum head and springs isn't going to happen.
It takes cutting the seats or longer stem valves with matching springs for that lift, or you could get that lift with the Hughes retainer set made for .620 coil bind specs and that set comes with new springs made for that lift.
Hughes Engines

The lifters just need a good inspection of the roller surfaces, outer shell and pushrod cups.
The rollers should all be about the same, as in how tight they are on the roller axle. (no notable looseness)

I have about 10 months on my 5.9 (probably 15,000 miles) with the original pushrods and lifters with the kit from Hughes and it's running great.