Spend a few days wandering the site since you're new to Gbodys. There is a ton of info and knowledge to absorb.
My '79 was much like yours when I got it. Very original with a 305 and TH200. Yours probably even has the same 2.29 gears, or something equally as lame.
Since you mentioned dual exhaust, check Rockauto for GN pieces. I got both over the axle pipes, muffler, and tailpipes for around $125. Then I did my own headpipes. Sounds great, in my opinion.
Unless your 305 is worn out, you might consider leaving it alone. For my money, regearing to 3.42s and swapping in a 200-4R made it feel like twice the power it had. Plus, since my cruise RPM stayed the same, I can still knock down better than 20mpg.
However you preceed, please start a build thread. We'd all love to follow along.
And welcome.