A little bench racing

I have a 440 combo with a solid flat tappet,with more cam, more gear, and more converter. With minimally worked 906s, it revved nicely to 6500, so I shifted there.
Then I tried 6200. It got quicker
So, i tried 5800/5900. Stayed the same.
So I've quit beating it up. As head limited as my combo is, rpm is simply not needed, or helpful.
To the op. First, I would try 5500. It actually might be quicker. I would bet it won't be slower. If it stays the same.... 5300.