I am also thinking of installing the VX cam and adjustable gear for a little more get-up-and-go.

It is a poor value. in my opinion, it is a compromised with meeting emissions standards, giving a torque curve that is ok with an automatic, which it sounds like yours is... just don't expect a big difference, but if you're getting it used, it likely is a better value. On a standard trans use an a, b or unitek mini sport.

The cam is used, so do I need to "break it in" in my engine?

Nope, that only applied to OHV pre 76 engines, not 76 -0n OHC type.

will it kill the engine with that many miles?

Hard to say, in itself no, but if it encourages you to rev the engine way up, yeah it could. Most internal strains on an engine go up with the square of engine rpm- so if you say 2000 rpm gives a "wear amount" of say 4, at 3k rpm it will be 9, at 5.5k it will be 30...though, a few other aspects are linear so at 5.5k rpm it will wear only 2.75 times as much. These are quickie rules of thumb, not carved in stone engineering kinda comments.

Should I take the head off and do anything?

Probably not, though there's always the chance of a head fatiguing off a valve stem at those high miles. Replacing all the valves with OE quality is pretty hard to justify though. I'd leave it alone, but keep your revs down.