That sounds like typical mileage if you have an Automatic, a manual should do much better. JBA headers are a PITA to install, having done it on my 4runner. But I must say the performance increase is nice. I've heard the Thorley's install easier but I don't know that from personal experience.
Keep the stock piping on the exhaust. Stock is about 2-3/8 and there is no benefit to going bigger on a stock motor. Welding in a new muffler to stock pipes is what I've done. Get a quality 2.5" muffler and it will work on the stock pipes. I put a big magnaflow on the 4runner and a big Dynomax on the Taco. Both are really quiet, the way I like it.
Other than that, keep the cats and O2 sensors where they are and run a tailpipe.
Tips for better mileage.
1. Stock Tire Size, non aggressive tread pattern makes a difference
2. Over inflate your tires, I clean fluids in the trans/transfer/diff
4. Clean the MAF and the throttle body
5. Easy on the accerator from stops
Do these things and you can get 18 to 19+ mpg
