how to achieve 600 to 650 hp sb 408?

Nokke said
yes thats correct what do you think is the cam too small?

To make near 650 yes, I would want at least .700 net lift and the heads that flow to that lift also. This means somewhere around 320 cfm @ .700.

You should really use a 2" header also.

This is my combo and I fully expect it to make around 660-670 hp.

Comp roller net lift .700 intake, 278 @ .050. Exhaust net lift .697 and 285 @ .050.

W5 heads 2.08/1.60 valves, peak flow of 328 @ .700 on the intake. 12.7 compression, 4" arm, 2" headers with 4" collectors. M1 tunnel ram with 2 Demon 750 TR carbs.