1972 Ford LTD My car makes a chugging noise

A backfire could be a leak in the exaust system and leaning out. The cuggin noise is probably due to either the emissions being disconnected and the egr plate under the carb used to get holes in it as well as the egr leads in teh intake get clogged. You probably need a carb overhaul though as the alcohol in the fuel has destroyed the accelerator pump as well as the diaphragm for the power vavle. 400's did have valve guide problems as well so it may be beneficial to pull the heads and get the valves ground and newer valve seats installed.