Try standing the manifold upright so the PB blaster will migrate down along the threads by gravity as you tap on the bolt head.
No promise this will work but it should increase your odds of success.
Is there any way to get a bit more leverage by tapping on the remaining flange piece so the bolt is moved with it? I've done this before with a cleat I wanted to remove that was mounted on an aluminum spar that was held in place with two stainless screws. I cut section out of the middle of the cleat then tapped on the horns. The screws came loose pronto. No more cleat.
TRUE GRIT
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