The first attempt would likely be to try drilling out the center of the bolt head and use an easy out tool. An easy out is essentially a reverse tapered tool that bites into the drilled hole as you try to loosen the bolt.
If that fails, then you would likely just cut the bolt heads off and then remove the manifold. You could probably get some vice grips on the bolt that is remaining and sticking out and turn to loosen. If you can’t loosen the bolt, then you may have to try drilling the center and use an easy out.
If that fails, then you probably have to drill the entire bolt out and then re-tap the hole with a heli-coil tool to cut new threads and insert a heli-coil thread insert.
I suspect there are lots of you tube videos of using easy outs and heli-coils. Hope you can get the bolts out one way or another!
