Does a original 75 dart swinger with a 318 have EGR?

You'll never know if the jet is working or not. Even if it was recently cleaned out and working properly, you would never ever know the performance difference. Idle or cruise difference either.

the EGR valve is only open when there is vacuum. So this means that at wide open throttle (WOT) the valve closes. The jets in the floor show no appreciable difference in performance (Low performance) when removed. If anything, removing the equipment will have the engine run worse since the carb is so set up and jetted for the smog equipment to be there.

The OSAC valve should be thrown as far away as possible, into a garbage can headed for China if possible.

The cat, if needed, should be replaced with a high flow unit, again, if needed. If it is not needed, you can live without one and the engine will run fine.

If your looking to upgrade the engine with dual exhaust, twin cats can be use without issue or any performance loss worth mentioning or noticing it missing.

Been there, done that, to many times to want to remember talking about. I have made this MOD to my vehicle and friends vehicle which all passed emissions tests.