If I were in your shoes, I would want to do a test install of that one screw BEFORE mounting the manifold back up.

Having not ever done this job yet on our white blocks, and only picturing the situation, screw sizes, etc., I would offer up the following which is just an opinion and subject to discussion/critique as usual.....

Run the correct size/thread tap down the hole just to clean things up a bit. Then insert the screw with a bunch of flat washers stacked up to the same thickness of the manifold, making sure the first washer in the stack (the one against the engine head) is fairly large area so that you don't leave an imprint in the aluminum due to the washer being too small. A flat piece of metal with a hole drilled in it would be even better, so that you distribute the load better. Then see if you can tighten the screw to the recommended torque without it stripping whatever threads are still remaining in the head. If the the theads are so bad off that they won't support the recommended torque, then you don't have much choice other than to drill and tap the hole out to the next size. Never increase the hole diameter any more than absolutely necessary. Drilling or rat tail filing the manifold hole may also be required. If you end up having to do that, you may want to mark the bolt head and the surrounding area on the manifold with some sort of bright paint to alert the next guy (maybe you a few years from now) that something wierd is up and not to mix up the screws.