i would spend more time thinking about the design to allow the expansion to happen with out cracking the runners, other than what material to use. ur proably looking to use sch10 or sch 40 weld bends. both of those are pleanty thick enuf for a turbo manifold. the amount of boost isnt relitive to anything..to many variables. mr burns himself said its not if an manifold will crack, its when.
another thing to consider is how those thicker weld bends are welded. alot of people dont understand how to weld them. they need to have 2 passes done, a root and a cover pass. you can pm if you have any other questions.