The 300 L6 is a good engine and can be made to make more power but the bottom line as usual is "how fast can you afford to go"! You can usually pic up a set of headers on ebay for these engines anywhere from $50-$100 but the real weak link is the carb and intake on this engine, Ford choked it to death with that 1 barrel, I think they did that to improve torque at the expense of HP. JC Whittney offers an Offenhouser style manifold to accept 390cfm 4 barrel for $189 but to be honest I think I would buy the Offy for same price out of the PAW catalog (pawengineparts.com). Clifford Performance is really overpriced stuff you can find elsewhere, PAW offers HiPo cams,springs,pistons,etc for the L6 engine. I talked to a guy who put a 240 L6 head on his 300 but he had to have some major chamber work done to lower the compression enough to make it work, guess I would have run a higher compression piston with 300 head vs spending the money on a big buck head job. I guess it really depends on what you want to do with your truck and how much you'd like or dislike to spend. After all is said and done you may wish you had swapped in a V8, there is a huge selection of "go fast" parts available for V8's but not so much for the L6's. Get it down on paper, write down the price of all the goodies you want to add to your 300 L6 and compare it to a simple junkyard jem V8 and you be the judge!
Hope this will help, Dan H.

 

my brother has a 300 inline six in his 77 one ton.It has a holley 4bbl and an offenhauser intake with a set of split headers. EVEN WITH the 4bbl he get's better gas mileage than my 90 Bronco. Power? He can lay rubber on the blacktop in that truck and it's a DUALLY!
Now, you can't say that that not impressive.

 

I never had a single problem with the carb on my 83 f-150, had about the same power as 302's, i stayed side by side with my buddy in his 85 302 injected, i miss that truck but oh well