I have no problem spinning a stock 350 block up to 6500 rpms for short burst. Even 7000 rpms.
Of course the cam and valve springs have a lot to do with how high the engine will rev.
A lot of times 5500 rpms seems like the engine is coming apart but that is due to the other stuff on the engine that is limiting the rpms (heads, exhaust, carb, intake, cam, valve springs, .......),
a friend of mine put an autozone 350 short block in his drag camaro with a good set of heads, intake, carb, 292H compcam, 3500 stall, 4.56 gear, and a 200 shot of N02. He ran it every weekend at the track for over 2 years and shifted between 6500 and 7000 rpm. the engine finally beat out a bearing but most likely due to the no2. I'm sure it would had lasted a lot longer with no n02 and a few less trips to the drags. and he drove that thing to work and street raced it during the week days. guy was crazy.
I had a similar experience with 350, 2 bolt, KB pistons, steel crank, arp rod bolts, and balanced. Nothing special but not stock. I tended to shift at valve float 7200 rpms. Used it in a 2600 pound car (240z), 3.90 gear, 5 speed trans (hit 7200 quickly). Ran it for 10 years and floated the valve every time I drove it. Also ran it at the track with N02.
So. I feel a stock 350 can handle a decent amount of rpm for short durations if the engine is in good condition. and the rest of the stuff on the engine is up to the task.
funny how a cam changes the feel of an engine. 4000 rpm feels very busy when the cam is small, but put in a big cam and 4000 rpms feels like the engine is barely revving.