choosing my bottom

DusterBoy15,
The pistons are the choice of the engine builder, the federal mogul and speed pro pistons are similar in weight and are a stock replacement style, and I would buy the lightest piston that I could if I was going to have to spend the money anyway, as they are similarly priced. The thing with the KB piston is the top ring lan is .080 higher in the piston and thus the need for a larger ring end gap as the ring is closer to the combustion space and will see more heat, so the expansion rate is higher. And yes this is why you have to file fit the rings per the engine bore.
As for the crank the 273 or the 318 will be fine, and the 360 crank is out due to the added stroke and the pistons that you are wanting to use. The KB 167 will get you a 0 deck with the stroke of 3.31 and decking the block. The later 318 rods are 340/360 rods so the weight wont be a issue. And as for the rings the gapless are ment for racing and not for street use and have to have a different bore finish to work properly. Also they wont last as long.