Tom954x4 and Harte3 are on the right track.
The 300 has a distinctive sound and it changes depending on which system you use much more than a V8. A 2.5 inch exhaust is plenty big enough for a stock or mildly modded engine and it will fit in the stock location all the way back. On a couple of my older Fords I had the exhaust come out in front of the rear wheel which I personally liked, but check your local laws.
No real mods on Nelly Belle, I went with low resistance Taylor spirial core wires, a good cap and rotor with brass contacts, Motorcraft plugs gapped at .050 vs .044 stock, and a 2 deg timing advance to 12 deg BTDC. I change oil every 5k with a Motorcraft filter and it burns about a quart during that time. I do all the other filters once a year along with PCV valve. During the summer this works fine but the winter blend fuel tends to give me a bit more spark knock. I did try .055 and 14 deg advance but it was too much noise for the old girl (over 200k miles) and me (over 60) My stock coil tests out fine so I decided to keep it but I have used Accel and MSD blaster coils in the past with no problems but they seem to have a higher fail rate than the Motorcraft. Since this truck is my daily driver and I'm on call a lot I wanted the truck to be as dependable as possible.
Your gas mileage is actually pretty good. I average about 16 to 17 during my 40 mile commute and have gotten up to 20 if I drive 60 mph. At 70 Mph Nelly Belle will drink a gallon every 18 miles. I can't really tell if my mods made any improvement over stock since it was long over due for those components to be replaced. When I hauled my 4500 lb rv I was pleasently surprised at how well it did. It hauled much better than my older 305s and 302s especially climbing hills and holding speed. I did use 93 octane and I actually got about 13 MPG which is 2 mpg better than any of my V8s could get hauling.
The Mazda tranny is not the best for hauling with its silly high reverse ratio but it did well. If I keep the truck, I will put a ZF from a F250 in.
"If women don't find you handsome they should find you handy" Red Green
Now-95 Black F150 XLT SC 300 5 Speed " Nelly Belle"
250 Chrysler Spitfire, 194, 250 Chevy, 223 Ford, 242,258 AMC, 225 Slant 6, 330 REO GC