The cheapest way to get there is probably with the 390 option and keep the magnum heads,... you didn't mention if the 318 was a roller block or not as that may open some cam options.The 408 edelbrock combo would meet your goals and allow for future growth should you go there.I would go the 390 route,upgrade your rods,clean up the magnums,air gap and 750 demon carb.You want some compression 10:1 and with the magnums you may get quench and pump-gas driveability.I am biased towards solid cams(maybe not for you)look at 560 lift and 250-255@50.With 390 cubes,some head work and a good cam 425hp should be easy.Hopefully others will contribute some more informed options.