My 79 has a hollowed out cat from previous owner, it sounds like the empty tin can it is.
What is the best long lasting hi flow cat?
Also it has the factory header (part of the thermal reactor I assume), would it be beneficial to replace that as well?

 

The 79 did not have a catalytic converter. That was the thermal reactors job that would be attached to the engine. I’m wondering if you just have a tired muffler . I’m not sure about heat exchanger issues.

 

Might be time for a racing beat system. Agree with the above, SA's did not have cats. Here is a picture of the exhaust minus the muffler. OEM exhaust parts are out there, finding them in good condition can be hard. Same with some of the gaskets.

 

Due to the availability of parts and the rarity of finding used parts in excellent condition, I did what Richard suggested on the 80. I went with a complete RB street port exhaust system. I don't regret it what so ever. Has a great sound and looks amazing. It's been on the car for 10 years now and has held up wonderfully! Here is what you can see in the engine bay:

 

I installed a MagnaFlow 99206HM after the RB header in my car. This cat doesn't have the extra air-intake port so if you need that you'll want to find a different version. For what it's worth, I need to take a look at it since my car has pretty much always had a terribly rich tune so it's probably gone. My exhaust is as follows; RB Header, Cat, Presilencer, RB Muffler. IIRC its all 2.5 inch pipe and it's still relatively loud. I do plan on one day revisiting this but it's what I have cobbled together and is quieter than it was.

 

Previous owner apparently put a cat of some kind on it. It’s hollow now. Would a magna flow cat handle the heat?