Speed is $...

"Not very fast"? Compared to what?
A stock 340 is a solid low 14 second car. Tune it and throw some gears at it and it's low 13 sec. or better.
And that is with points ignition, a carburetor, and exhaust manifolds.
If yours is not performing at least on a par with that, you've got other issues that throwing more parts at isn't going to solve. As the previous posters said, check your tune.
Remember, EFI is not going to perform better than a well-sorted carb. Drivability may improve, but AFR is AFR regardless of what mechanism is delivering it. MSD may be flashy, but any improvement on a stocker is dubious at best. The headers are only a slight improvement over the quite-good factory Hipo manifolds on an otherwise stock motor. You basically have a stock motor with some expensive toys on it.
New heads? Your cam, while quite good for a street cam; is not even taking full advantage of your stock X heads at this point. Upgrading the heads will serve no purpose other than to lighten your wallet, until the rest of your combination calls for it.
As said, triple check your tune, then go for gears and/or a convertor (if an automatic). Then sort the suspension and get it to hook.