cam, cam, cam
oh, and heads
but mostly cam
did i say cam?
bore it out between 30 and 40 over, find the perfect cam,
then find the right heads and then on to rockers and springs and intake manifold and headers, and high performance aluminum water pump, high volume oil pan, high volume oil pump, high volume oil filter, here's where you buy a charger of ur choice if goin' all out, if not then on to picking a carb, two carbs if you wanna be then man and punch a hole in the hood, and air filter device of your choice, cold air is the only way, wether is above the hood or up in front, since ur buying a kit, find out if it's internally or externally balanced, if it's externally, you'll need a harmonic balancer for a 400, and the flywheel as well, then go all out and dress up the rest of the engine bay with high performance or high cosmetic.
Why go through the motions? Just buy a long block at the dealership and be done with it. If you live near many dealers, check prices or work a deal with any dealer that is accommodating. They also have a limited milage warranty that comes with the engine.
But first, go through the paper chase of each part needed to build a short block. Add machine work also, i.e. , valve jobs, bore jobs, and new cam bearings installation. Then see how much they want to mag the block for cracks and flush the block clean. Keep adding more to the list for a new gasket set, new water pump, new oil pump, and new crank/rod bearings. Buy a service book that has the tolerance spec's, as you'll need to measure every part going back in the engine.
I've missed a lot of things on the list, but this should get you to the counter for a little surprise at the cost of individual parts as opposed to a long block in the crate. I think in the long (no pun intended) run, the long block will be more economical.