It is not just an RPM thing. RPM will increase loads/force on the crank while lighter rods, pistons and rings will reduces these. They will also increase with higher cylinder pressure. Also stroke and rod length do come it play.
fwiw, before the blower, i spun mine to 6400 on all stock stuff on a few different occasions, trying to take advantage of the cams. i was shifting at 6200 most of the time though
Keep the oil filled to max at all times.......I spin my to 6150 all the time and the car has 90,000 miles on it now.....I am doing heads and cams now and plan to spin to 6400 every time I run! From what I have been seeing 6500 can be safely spun with the stock block as long as the engine is getting oil and you got better valve springs........
The biggest enemy of a crankshaft is deflection, the constant twisting wavelike motion that the crank experiences. RPM is not nearly as destructive.
Deflection is largely dependant on 3 load factors, peak cylinder pressures, rod/stroke ratio, and stroke. The greater the cylinder pressure and stroke, the greater the leverage on the crank(rod journal) the connecting rod has. Rod/stroke ratio also plays a role as the friction on the thrust side of the cylinder wall, and again, the leverage is affected by the rod/stroke ratio. A greater angle (smaller ratio) will be present by using a shorter rod or by increasing the stroke. A reduced angle (larger ratio) will be present with a longer rod or a shorter stroke relatively speaking.
We have exactly a 1.674 rod/stroke ratio, which in the grand scheme of things is a significant angle.
With that said, I think you should be fine on your power levels.