It is always hard to say so go and test it. Having said that its an old engine, sure you have a few bits but they are not the main source of extra power -- its always in the heads and cam -- the rest plays second fiddle. So my guess would be 140 HP at the rear wheels at the very best.
The horse power measurement used in 1967 was gross horsepower. This was without even the alternator, waterpump, or anything else hooked up to the engine. The exhaust was litteraly sucked out of the engine. This was number that could never occur in any car with the engine installed. This led to the introduction of the net horsepower number introduced in 1971(?). So that 200HP would have been dropped to probably about 180 or so. People also tend to underestimate how much losses there are in the dirveline. A for real number: a 345 HP 302 Ford Crate motor installed makes about 255 at the rear wheels. This is from dyno testing, and some dynos are more optimistic than others.
Non of this will make your car faster or slower than what it otherwise might be. My experience is that headers, intake manifolds, mufflers and so on make very little measurable power on a dyno or dragstrip. They do make the car sound a lot better, and that makes it "feel" faster.
You need to identify what your really after and what budget you have to make it happen. The key (as I said previously) is the heads and cam. Its best if they can be matched for each other as the combination determines the result. You can have considerably more performance without a reduction in fuel consumption (unless you have a very heavy right foot all the time). Modern replacement cylinder heads are far more efficient and hence fuel consumption remains good. In addition, modern cam designs, especially roller cams, are far better than the classic cams of the day.
So here is a basic formula assuming a mainly street driven car (no guarantees);
1) Heads - Twisted Wedge 170 (good to 400HP). Make sure the springs match the cam and use good valves etc. Alternatively, Canfield 195 (good to 500HP) which leave more room to move, e.g, when going to a 347 crank. Cost around $1400.00 for a pair. With a 302 you may consider dropping chamber size to 58cc and if staying with the 289 crank smaller again say 2 more cc less.
2) Cam - Hydraulic Roller. Look around the 1/2 inch lift and 220-240 duration at 50 thou range with lobe separation around 110. Best option is to get a custom cam. Cost $300 - $500
3) Sundry bits eg Gaskets, roller rockers, push rods, lifters $500.00
Result 330 - 350 HP at the crank.
For me the actual figure is not important. Its the fun and thrill from the felling of being pushed back into the seat when you hit it. Obviously this can lead to wanting more and more takes more dollars and time. So you want close to 500 HP - well a 347 stroker kit to start and etc etc etc.