One thing you can try to help out, once youve got it bolted up, take a torch and heat the pipe near the manifold. There is usually a bend hear the flange, try and heat at that bend. (assuming you have acess to a torch)

If there is some sort of stress in the pipe, the heating will take the stress out. youll actually see small parts of the pipe metal flake off as the pipe "un-stressess" Many times while working at a Car-X we would heat certain new pipes to relieve stress. Aftermarket pipes dont always fit correctly. Some would be stressed so much, they would crack off in a few thousand miles.