Magnum Camshaft help?

May sound like a dumb question, but magnum block what? 5.2 or 5.9?

Compression is something you'll need to measure for. Even with the later model magnums, pistons were still a little ways in the deck and the only way to get anything closer than a ballpark is to break out a burette, clay, and piece of acrylic. There's a good thread in the how to section...detailed and a little of a read, but it'll give you everything you need to know to accurately know your static compression ratio.

All that being said, I'm going to step on a limb and assume you have a 5.9...with a stock bottom end advertised at 9.1:1, and 58cc chambers (assuming the advertising is accurate) should* put you around 9.5:1

Otherwise, longevity of any valvetrain is based on the correct matching of parts and operating within their intended usage.