The last time we built a Miata-powered MGB we used a mild steel ANSA exhaust system mated to a Miata downpipe. The owner of this GT liked the sound so much that he chose the same system. Unfortunately for us, ANSA (previously know by the brand name Monza) had again been sold to Pace Setter. In the ensuing years the dies or manufacturing techniques have changed this system did NOT fit very well at all. As this car is due to be sent to California, we needed to fit a CARB-approved catalytic converter to pass the smog tests.
We sourced the smallest catalytic converter available with our pipe sizes and went to work making a custom downpipe that would mate to the stock Miata exhaust manifold. In addition, we had to fabricate inlet and outlet pipes to mate to the Pace Setter exhaust behind the catalytic.
Using welding rod as a gauge was a very effective way of identifying the proper shape of the downpipe.
Cut, fit, weld. Cut, fit, weld.