I have revved stock bottom ends to 6500 RPM and a little more, but, that was only when the top end was doing its job too. You've got Windsor Jr. heads which don't typically flow enough to warrant much over 5600-5700 or so, ported, - unless they have been severely ported. Even then I don't think the runner volume will be what you need. How about your springs how are they?
I would guess that motor will peak around or before 5,500-5,600 so there my not be a lot of sense in going higher with the RPM. Dyno it and see.
What year was the block, that will tell us if the pistons are cast or forged, although if around 5,600 RPM is what you need it won't make a lot of difference.
Best,
Ron