Pick up a set of magnum manifolds and point the outlet forward and down. No need for headers because the manifolds will support all the power you'd ever need. You can fab up your crossover from there, using no LARGER than 2.5" piping. Believe it or not, 2-2.25" is optimum since the single biggest restriction is the turbo itself, and a smaller pipe will help spool. Race cars making 1500+ hp run a 2.5" crossover so keep that in mind.
After the turbo though, you want a big pipe. Depending on how much power you plan on making, 3" (single) would be absolute minimum.