/6 single or dual exhaust?

I picked up a very nice grandpa 1975 Dart Swinger special with 37,000 miles. My 15 year old son and myself are building this as a daily driver for him. We already have a 1975 Duster lmost done that will be 90% track. Even though the 75 swinger is getting a few upgrades such as super six 2 bbl set up, a 8 3/4 3:55 posi rear end I have extra new paint and the heads milled down .050 which should put me at about 9:1 compression. Well now that you have the facts, I plan on using a 2 1/4 pipe. Should I have them run a "Y" off that and use 2 cherry bombs, Run a sinle pipe into a flowmaster that has 2 exit pipes, or stick to one pipe straight back with a thrush or something simular. I am keeping the original exhaust manifold and have the matching 2 bbl. I do have a set of headers cant find a name on them, put a sticker with part number 10-500? I also have a Aluminum super six welded intake. i can see some hailine cracks in the bowl, but it was pressure tested and is fine. I am hoping to still get about 18-20 mpg. What ever parts I dont use, I will be selling along with an edelbrock RPM preformer for a small block v-8, with a holley avenger 670.