Sounds pretty good to me as well. Head work and a cam will get you closer all things being equal, and cheap. Make sure you don't neglect the valve springs when getting head work. Exhaust side will probably get you more bang for the buck than intake, if you had to choose where to spend the money, but at least bowl porting both sides should help things out. Marking the gaskets on the heads and not going over that size and doing some of the work yourself could help too, just think about the "flow" if you can't afford to have someone do it for you.

my van weighs 4450 lbs with driver, makes 340 whp N/A, and runs a best of 13.70 @ 99.8 mph with only a 1.9 - 2.0 60' time (my converter needs to be swapped!). I run Eddy heads (box stock other that minor clean up of machined corners and machining the push rod holes out to clear the angle of the roller cam/pushrods in my old LA engine, which shouldn't be a problem in a magnum engine. My cam is the crane retrofit #4 cam, 112 lsa, 107 installed intake c'line, 230/238 @ .050, .528/.548 lift, an edelbrock RPM airgap intake and Holley 750 VS carb. Headers, etc. for everything else, so the potential is definitely there to make 400 hp without too much work. Oh, i have 10.2:1 compression also. It operates my power brakes fine on the street too Not sure what my vacuum @ idle is off the top of my head tho.

Good luck!