Which exhaust/exhaust system would work best for my car?

Why are you not asking an auto technician about this? We have no idea as to the condition of this FORTY FOUR YEAR OLD car. Flowmaster is not an "exhaust" system, it is the name of a company that manufactures mufflers. The exhaust system starts at the engine and ends up in the tailpipe. There is no "best" of pretty much anything. This is a marketing term. For sure there are cheap and inadequately engineered parts and products. But "best?" Go find a technician to learn the condition of the exhaust. It makes no sense to place a new muffler on rusted and damage exhaust components. Why the "roar" syndrome? This a photo of your year, brand and model car. It has been restored to perfection BY EXPERTS. The mechanical components were restored or repaired by mechanics. Go find an expert. Sharp looking designs, almost Chevelle style from that era.

headers are a plus, flowmaster 40 series is what i use on everything, turbo mufflers sound good untill you put your foot in it then they sound like glasspacks. 2 1/2 pipe is big enough. on my 440 road runner i used 3 inch and dumped it at the rear axle but interior resonation is bad, so i recomend going out at the back. and yes there are plenty of us who know about 40+year old cars. oh and by the way i have a 70 torino that is my next project

Does it have single or dual exhaust? If you have single and want dual, everything from manifolds back needs to be changed. Go about 2 1/2" exhaust tubing all the way, it'll sound sweet. Don't forget to re-adjust carburetor to better flow, need to richen the air/fuel mixture.

i generally build my own, and i would use 2.5" and install a nice oversize balance tube between the 2 sides. not much room under there though if you notice my avatar. i also use flowmasters when i build an exhaust system.

if you have single exhaust you might want to consider running duels ! go from the exhaust manifolds using 2 1/2 inch pipe and if you want it loud use cherry bomb mufflers but if you just want a mellow sound then go with turbo mufflers !