I must be missing something here...

You must mean getting you're current 440 to 350 net hp?? The 68-70 440's were 375 hp. The 440 six pack's were 390hp and I think 490 foot pounds of torque. Yes the gas mileage isn't gonna be very good,but that's the joy of having a big block.
A lumpy idle? You don't have to overcam a 440 to make power. Cam technology has come a long waysin the last few years

I don't see a 5.7 HEMI outperforming a nice healthy 440 without a bunch of mods. 440's are reliable horsepower

I'd build a 451 or optimize your current build to be a torque monster and run a set of 3.23 or so gears.