Classic/Antique Car Repair: one new muffler or two?, path of least resistance, flowmaster muffler

Question
hello, my 1965 Lincoln Continental needed a new muffler on the left side so i ordered a flowmaster muffler to fit that side, and i was told "flowmaster only makes high performance, so if your other muffler isnt the same kind, you will have issues with pressure differences" is there any truth to this, do i need to order another muffler of the same for the other side? thanks for the help if you can!

Answer
What you have been told is true, now let's apply it to your car. Do you have two seperate exhaust systems or is it joined and then reseperated? If they are seperate, it will cause a backpressure imbalance on your valves. If they are tied together, you will have the flow taking the path of least resistance and not getting full benefit from either muffler.

If it were me, I would just order another one.