I found that you need extra gobs of silicon on the ends of the manifold to help it seal better. It sounds like you got at least 450hp. What is your rpms at 50mph?? Mine are sitting at 2363rpms @ 60mph and 2000rpms @ 50mph with my .83 ratio 5th gear. Just curious on what cam your running in that beast.

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347 Stroker/XE284H(284/296 dur, 240/246 dur @ .050), BG 650 VS Speed Demon, Ported Victor Jr intake, Jack Roush World Product (ported from 180cc to 195cc) Jr Heads(upgraded to 2.02/1.60 valves) 260cfm/190cfm @ .550 lift, 11:1 comp... 450-475HP