543

The TF heads aren’t in this particular discussion because....... oldkimmer doesn’t own any of them.

Depending on the rest of the combo, with your original style stage 6’s on a 540” motor...... I would expect it to make peak power somewhere in the 6000rpm range.
If it had enough compression, cam, header, intake, and carb to get it there.
So, yes...... the top end of the “usable” power curve would be in the 6500 range.

Of course, there is really only one way to know where any given combo of parts will actually make peak power.

On a bigger cube combo like what’s being talked about here........ I would expect a properly ported set of SR’s would make as much power as the TF270’s.
I think the same could be said for the 285cc MW Chapman stage 6’s.
In fact, they’re probably the best of the three.