The OHV 4.0 was always 165/170hp. The SOHC began as a 200(or pick your advertisement number)hp engine. It didn't change except for small intake(manifold, not damn air cleaner) improvements.
The biggest change will come from a tune. The SOHC headers are wonderful as stock parts. Don't waste $500 on headers that may add 5hp, get the flasher and a custom tune with it. That will gain the most. The UDP(balancer) is the 2nd best gain. The exhaust(cats and back), an electric fan, and a better air cleaner are minor upgrades. Do the PCM tune last to take advantage of the improvements.
Stop believing everything that you read in a book, a website, or a manual. This forum is full of much more accurate information, if you can get past the stuff posted from guys reading from erroneous books/websites/manuals. Books are not absolutes, they are made by humans, usually not caring how accurate they are. Take them and like any information, verify it before using it as facts in your own words. Regards,