Mustang Monthly magazine recently did an article in which they took a stock 289 2v and added headers and dual exhaust to pick up the horsepower from 141 to 153 at the rear wheels, a gain of 12 horsepower.
The next month, they swapped the stock 2-bbl carburetor and stock 2-bbl. intake for a stock 4-bbl. Autolite carburetor and cast-iron 4-bbl. intake. The results were 18 more horsepower at the rear wheels (carburetor, intake, headers, and exhaust had combined for a total of 30 more horsepower at the rear wheels).
They then swapped out the stock, restrictive air cleaner assembly (which isn't that different from the stock one on our 302s) for an open-element 14" air cleaner, pickin up another 15 horsepower at the rear wheels, taking the total to 186 rear-wheel horsepower.
What does this all mean to you and me?
Well, for starters, our 302s with their low compression and smog gear aren't as powerful as that stock 289; but they benefit from the same upgrades. Also, you and I aren't likely to be going with a stock 60's era 289 intake, we're more likely to use a street-performance intake from Edelbrock or Weiand that runs cooler and builds more power. While you might not see that same 186rwhp number they got out of their 289. You'll definitely see similar overall gains, if not a little more, from the carb, intake, headers, exhaust, and air cleaner improvements. You might even see a little improvement in your gas mileage if you drive it conservatively (but what would be the fun in that?)
If you were really feeling ambitious, you could throw a better pair of cylinder heads and a little lumpier camshaft in there and make some serious gains; but ideally, replacing the stock low-compression pistons and a full rebuild will get you the overall best results from one of these engines. How far you go is only limited by your abilities and/or pocketbook.
__________________ 1976 Ford Mustang II Ghia: 302 with a 600cfm Holley, Edelbrock Performer 289 intake, Dynomax Blackjack headers, and 2.5" exhaust with Flowmaster Super 44s. C4 Automatic tranny, chrome 16" pony wheels, fuzzy dice, half-vinyl top, and painted in the tackiest color ever (harvest gold, that's why I call it "The Goldenrod").
Also have a 2003 Dodge Ram (lightly modded daily driver/tow rig/office on four wheels/dining room/home away from home/workshop... I call it "The Big Blue Dawg".) | 
