Thats the awesome thing about these Magnums, they're rock solid. The rockers are easy enough. Remove the valve covers (clean as much as you can around them first or stuff will fall in when you remove them) them you're there.
This is the bit from the service manual.
(1) Rotate the crankshaft until the “V8” mark lines up with the TDC mark on the timing chain case cover. This mark is located 147° ATDC from the No.1 firing position. (2) Install the push rods in the same order as removed. (3) Install rocker arm and pivot assemblies in the same order as removed. Tighten the rocker arm bolts to 28 N·m (21 ft. lbs.) torque.
CAUTION: DO NOT rotate or crank the engine during or immediately after rocker arm installation. Allow the hydraulic roller tappets adequate time to bleed down (about 5 minutes).
Some guys say to just tighten them down until they are snug, but i've never done it that way. Then again, most engines i've rebuilt have actual lash adjustment. I've only done a few with hydraulic lifters.