My idea for an engine was much like yours. For my BEC GT40, I built a 1965 SBF 289 (stronger-higher nickel content block) stroked to 302 (i.e. for 5 litre FIA limit) with a flat tappet split-duration cam, roller rockers and webers. Puts out ~400 hp&torque. It has been a good choice for me.
Larry