I have the same setup (L29/blackbox), but my internals are all stock. I've never had mine to 4500rpm, and rarely get over 3K. So, I can't really help you with the higher revs. But, I've looked through MY tune, and I too can't find anything that would kill it at a certain rpm. Speed cutouts are there, and highly effective, but nothing that really points to "set this to x, and you'll rev to y".
I did consider doing the 411 swap before burning up a VIN License on my black box. But, for me I didn't think it would be worth it. My truck gets maybe 5k miles/year. Its not a show/race/high speed anything. I'm tuning it to get the best mpgs I can out of a thirsty BBC. If I happen to get better performance so be it, but that's not really what I'm after on THIS rig. Down the road....who knows. So I can understand you wanting to stick with the black box ecm. Hang in there, you'll get 'er figured out. As for the 411, most junk yards have 'em for dang cheap, and they'll usually let you take the connectors/harness if you ask nicely. I've know guys to do the whole swap for less than $100.