Two things I ran into when working on big gas powered & high mileage trucks in the old days was worn throttle butterfly shafts in the carb which caused hard to find vacuum leaks. Also made sudden throttle changes act like a bad accelerator pump. Another was worn bushings or shaft in the distributor. Usually timing & dwell would check out ok at idle but would go off as the rpm’s increased . Just a simple wiggle test will often let you know if there’s a problem in those areas. If the exhaust check doesn’t pan out, check the above items. Another possibility is exhaust lobe going flat on the cam, causing the intake back fire. Just a couple ideas to check out if needed. As an fyi, I’ve always been a fan (admirer?) of the 348/409 engines.
