How many RPM's is the 500hp rotating assembly take reliably? I'm ordering a kit that calls for turning up to 5800 but I don't want to be going through the bottom end a season or two.

I think you will be fine. Unless you hold it at 5800 rpm for miles and miles. I will say this I don't think the stock oil system will be up to the task as far as cooling goes.

You should be fine. Guys use to spin the chit out of those stock Chevy cranks and 7/16 rods for many years.

If the crank and bearings are good and the oil don't get to hot you will be fine.

I had a built 500EFi (575hp) Stock crank & Rods in a 271 fastech. Ran 17 miles @ 5400 no problems.
A guy named Saris put it together & dynoed it.