I think exhaust temps would make short work of oily string.....
Swapping in a new pipe onto distorted and rusty flanges isn't ideal. Everything needs to be clean and flat and the end of the pipe needs to fit properly against the manifold. Exhaust gaskets do seem to be a bit thin mow days so can't take up the slack very much. A light smear of any good exhaust paste should help but fitting new studs and using new nuts would give you the best chance of keeping a good seal. They don't have to be done up super tight, that tends to distort things but frequent checks on the nuts for the first few weeks is a good idea. If it starts to blow, check and tighten immediately or the gasket will burn and you'll have to start over.

Have you got the steady bracket running from the bell housing to the exhaust pipe? Having the pipe clamped to this stops the pipe moving and also reduces the chance of the manifold cracking, a common B16 problem.