here's how the lower manifold looks after a bit of radiusing:
Enough of that. Next tool...
meet mr. FleshGrinder... a metal saw on an angle grinder, goes through half an inch of aluminium in a second without effort. The most dangerous tool in the garage.
First cut with the grinder. The edge will be radiused right down to the edge, so the missed cuts will be completely removed.
Opened a bit more. The idea is to remove the wall so the two chambers are joined seamlessly.