I think you need more cam. I am using the xe282hr cam in my 393 (387 actually), and the power starts to fall off way before 6500, more like 5500. However I have 3 things working against me in the rpm department:
1) Tiny headers (1+5/8).
2) Stealth Intake
3) Old Windsor SR heads

Depending on your tire height, you are going to need some rpm capability at the far end of the track. A 26" tire and 125MPH you will be > 6500 rpm

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65 Fastback:393W, G-Force T5, 4.11:1 Moser N-pro case / 31 spline axles, 4 wheel Disc (low 11's)