Why not supporting environmental protection with our historic cars? Here is an easy and cost effective way to integrate a catalytic converter into the exhaust system. I use the Tenneco Walker Mini Converter. I has a diameter of 55.5 mm and was planned to use in the Audi Coupe 2.3l 5-Cyl. (Walker type no. 52590-03W). Other diamerters (42.5 mm, 45.5 mm, 50.5 mm) are also available.
The converter is intergrated in the exhaust pipe just behind the 2 in 1 tube of my CSC sports exhaust system with stainless steel pipe clamps. Sealing compound is recommended. It should be mounted as near as possible to engine's exhaust ports.
There is no special engine control system. I run stock Weber 40 DCOE carbs. Ok, this is not very effective in terms of emission reduction but better than nothing. I can't measure any significant engine power reduction due to the streamlined sheet metal catalyst technology.