If I use them on a manifold with a crossover I roll up some aluminum flashing into a log to fit into the crossover from one side to the other and put daub of epoxy over each end of it. Just to where it catches the end of the crossover before where it opens up at, and then cap it with a square of flashing. Then I fill it to the manifold flange with a mixture of epoxy and coarse steel wool, usually it's about 1/2 to 3/4 deep. After it fully cures, file it down and block sand it smooth. The right way would probably be to weld up the crossover at the flange but this method is more easily reversed if you decide you need exhaust heat in the manifold.