Another excellent way to cut aluminum is to make a pattern, and follow it with a carbide tipped router bit
if you get a bearing bit, you can fuly radius parts, and they come in all sorts of fantastic profiles
the bits are relatively cheap and last quite a long time (feed rate has to be slower than i was typically used to)

execution on the manifold lacked a bit of forethought, but the radii came out well, and they were cut with a regular "wood" router