That 429hp seems awfully high for a 5.0. I just saw an article on a XE274 cammed 302 with some flowing Dart heads that pushed it to around 360hp. You sure it wasn't a 347 Stroker motor, would make it more reasonable power on the table.

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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 Jr Heads (ported from 180cc to 195cc)(upgraded to 2.02/1.60 valves) 260cfm/190cfm @ .550 lift, 11:1 comp...309rwhp for now:):)