best all around leaf springs for 66 barracuda

Depends on what your goal is.

Super stock springs are great for drag racing, but your car won't sit level side to side. Or handle turns the same way right/left, since the SS springs have more arch and an extra leaf on one side. They'll also lift the back end of the car quite substantially, which will have a negative effect on cornering. SS spring rates run from 140 lb/in up into the 160 lb/in range depending on which ones you get. All too stiff for use on an A with a rear sway bar IMO. So, good for drag racing, but lousy for handling applications.

Espo makes good stuff, but you'll have to call them. They can also alter the spring arch to help set your ride height, I believe they will do -2", -1", +1", and +2" springs. Not sure what the actual spring rates are though.

I have no idea what Firm Feel is selling for springs at the moment. They used to sell the Mopar oval track springs, and they sure didn't cost $500.

You can still get the Mopar oval track springs, part # is P4529414 for zero arch 120 lb/in springs, P4529415 for +1" arch 120 lb/in springs. Don't let the oval track thing concern you, the spring doesn't care where its run. It's just a spring with a reinforced front section, which is good for improved handling. Both Summit and Jegs sell the mopar oval track springs.

I have AFCO 20231m's on my Duster. It's a 121 lb/in spring with a reinforced front section, much like the mopar oval track springs. For that matter AFCO sold them as oval track springs. Unfortunately, it seems that AFCO has at least temporarily stopped making that particular spring. Which is a shame, because they work great. There has been some speculation that the AFCO 20231M is actually the same spring as the mopar oval tracks, and because the Mopar parts are being made again the AFCO's aren't. Not sure if there's anything to that. I'll probably be buying a set here pretty soon for my EL5 dart, I just have to decide which arch to get for it.