built up 318 oddly low MPG's? hp rating?

A vacuum secondary carb is much more reasonable for this combo, and should be pretty close out of the box. Try putting the jetting back, and make sure the power valve is 2 numbers below your cruise vacuum.

You can't (or shouldn't) just install new pistons in existing bores. At a minimum the bores would need to be honed to seat new rings. For best results the bores should be resurfaced to the next oversize. If the engine has minimal mileage since the .030 resurface you could probably get by with a light hone. In any case I wouldn't personally attempt this with the engine in the chassis, but I suppose it could be done.

Kieth Black KB167 pistons have a taller compression height than typical replacement 318 pistons. They are popular because they bump compression about a full point. With 318 heads, especially closed chamber 302 or magnum castings 9.5 compression is pretty easy to achieve. You could probably get 9.0 with the 360 heads, but some milling may still be required. Before the KB167's showed up, the domed "11.5:1" pistons were popular. With 360 heads they would yield 9.5-10.0 compression, albeit with a less than optimal chamber shape. During the 70's and 80's that was a pretty common way to go.

Heck, as long as you're gonna replace the pistons, why not replace the crank too. A 4" stroke crank would bring your displacement to 390. Then the heads and intake wouldn't seem quite so large (but the cam would seem kinda small).

Skip the MSD unless you really think you have an ignition issue. There's nothing wrong with MSD, but it ain't magic. There's nothing wrong with Mopar ignition either. Both are adequate. But if you do have an ignition problem fix that first before monkeying with the carb.