Chuckie's right, its the application of heat that will ultimately kill the welds. A manifold would benefit from being heat treated because it relieves any residual stresses created in the heat affected zone (HAZ) by the welding itself and thus making it less brittle. There are better&cheaper ways to prevent common cracking, i.e. elongate the bolt holes in pairs, starting at d, d + 0.06", d + 0.12" etc. This will allow the manifold to flex/expand whilst maintaining a seal, taking any strain off the welds themselves which is especially important for s/steel and less important for DI.
Go at least 413 s/s as thick as you can get it. DI is heaps better but its costly mainly because of the cost of tooling