First off, what are your goals for this build? Seems like it's primarily a street engine, but how heavy of a vehicle are you putting it in and what gearing are you going with?
The 413 is just a 440 with a smaller bore (with thicker walls) so aside from the big valve heads, all the same 440 parts with work. Plus you are already starting with a steel crank engine. There are off-the-shelf pistons being made, I believe by silv-O-lite, and a few others, but you may need some lead time on getting them produced or you can hit up any custom piston manufacturer to get some made, but you'll need see how much overbore you'll need first (among other things).
Also, you may also want to find some later 915 or 906 heads to rework and set the ones you have (which have smaller valves and the funky aluminum rocker pedestals) aside. The newer heads will not only be less expensive to rework, but the more conventional rocker set-up and valve covers will be far easier to source and work fine with your dual quad manifold. As for cams, the factory (non Max Wedge) 413 stuff was very conservative with ~.430 lift and low duration, so stepping up to the 440 Magnum cam or even something hotter with .470+ lift and more duration would be a step in the right direction. I strongly suggest contacting any of the big Mopar aftermarket cam techs for recommendations, like Howards o Hughes and they can recommend what will work for your goals. Once you have a cam selected, you'll know the compression you'll need to be most compatible and can then work with the heads you decide on to get the right pistons to achieve that compression number. Once you're down the bare block and have a machinist inspect it you'll know what else the blcok will need and the overbore that will be necessary.
413's are pretty awesome engines that don't get as much respect, but were (and are) powerhouses when done right.
