To answer your question specifically:
"All the cams that you suggested are marked as idle starting at 1500 through 5500 (roughly). What exactly does this mean?"
This is the general powerband of the cam (not the idle RPM's). Generally the cam makers are going to rate this against the standard 350 ci. So on your 327 these values will shift to a higher RPM range (and 383 strokers and 400 SBC will shift lower).
Also, is that cam you initially mentioned a used cam? I would never trust a used flat tappet cam. Even if you kept the lifters sorted and mated them back on the same cam lobes, you are looking at a 50/50 chance of wiping the lobes on flat tappet cam. And if you didn't keep track of your old lifters and what bores they went to, forget it, game over. Also when you buy the a new flat tappet cam you must get new lifters too. Never re-use your old ones! I would suggest getting the springs too, but you are getting a set of pre-assembled heads, hopefully the springs are close enough to the cam makers spec.
Make sure you read up and study the proper way to break in a flat tappet cam. It is absolutely critical you follow the correct procedure. It is also important to chose your normal engine oil to match the demands of a flat tappet cam. Oil these days aint what it used to be back in the day.