Performance L6

If I were to build a factory 200ci factory I6, there are really only 2 things I would do to it:

1. Swap in a T5 manual 5 speed transmission, this mod alone makes the engine feel a lot peppier according to everyone who has done it.
2. Turbocharge it. You can get pretty easily get more than 250HP out of the engine that way...there are of course quite a few supporting mods that go with turbocharging, but you can retain the carb in a blow-through configuration...you just have to modify the carb for boost("Maximum Boost" by Corky Bell is a great read for turbocharging anything and even has a chapter that deals with carbs and modifying them for boost)

That being said.....I wouldn't bother. If I wanted a performance I6, I would swap in a BA/BF Falcon I6 from an Australian late model falcon...those are distantly related to the original I6 motors but far more evolved...you could swap in a BF motor for less than it would cost to turbocharge the original I6(if you go with the 250HP NA version and not the 350HP+ XR6 version).

The most sensible and cheapest way to get more power though is of course the SBF v8 swap....its a well-known path that you will be able to find plenty of support for.

I have a long history with the 200 and its bigger cousin the 250. BTW: The 200 to 250 swap is more complicated than a 200 to V8 swap.

First make sure all maintenance and safety items are up to date. Brakes, including all 3 brake hoses.

Your best bang for the buck is a ebay stainless steel header and a single 2 1/4 or dual 2" exhaust. Do this mod first.

You have a spark-o-matic carb and a load-o-matic distributor. Both are poor performers, but you must have both or neither.

Install a 68 200 distributor and a 2 into 1 carb adapter and a Weber 32/36 carb. Vintage Inlines has the carb and adapter. RockAuto has the distributor.

Exhaust and carb will run you $1000-1200 depending on how much the exhaust costs.

Thats the best you can do without removing the head. The exhaust will add a few hp and the carb will tune much better and give you a smoother idle and acceleration with no flat spots or hesitation.

Down the road look for a C9 or a D8 head. The D8 is preferable and easier to find. They were on Fairmonts, Granadas and Fox Mustangs for a couple years.

The key to making theses sixes perform is keeping them tuned and light weight. If you start adding a bunch of weight, the steering, stopping and performance goes right in the toilet.

Having built a 347 stroker V8 as well as a 200 L6 both in 65's, you'll get 10x the attention at a show with a souped up 6. EVERYONE swaps and it's boring after a bit. My advice, get a basic rebuild of short block, balanced and blueprinted and shoot for 10:1 compression. Classic Inline makes a sweet aluminum head to look into or, if you want to keep a "stock" look, get a 250 head from latest model you can find, take it to a good shop and shave it down, polish the bowl and what they can get to behind the valves, bigger valves with multi angle job and have them braze in the 2V adapter and put a holley 2V on it. Get a good cam (i went solid lifter b/c SOOOOO easy to get to) and add the headers and you'll have a snappy car. 5 speed will free up a lotta HP, too. I went with a triple weber synced carb setup. Looks awesome but steep learning curve.

In response to your post I will reiterate what several others have posted. For me first& foremost safety upgrades 1st brakes as the brakes on the 4 lug mustangs & small tires suck even if every componet is new drove a 66 200 3 speed for a few years still have it & i have a 67 vert now 200 auto stock brakes on both were terrible. Upgraded the 67 to power disc 4 lugs still on car. On the 67 installed a msd 6 al ign. system ,dist. to fire the ign & a 2300 holley sniper efi system. At this time have not did anything else to the stock eng.Does it run like a v8 no but the difference between before & now is like day & night personally I think these 2 items are one of the best mods for a stock 200 . Cheap no but no major eng work or tuneing to get everything to work together. Should you choose the holley route save your self a lot of trouble buy the kit & a tank with a efi pump in it . Just know this when you start one thing leads to another to another. Pick your projects & enjoy the car for what it is something that a lot of old & young people wish they had one like.

Yes- a modified and warmed over 6 is does get attention but that attention comes at a price. Expect to pay a lot for a little gain. Two relatively cheap mods (depending on your level of mechanical ability) is swapping in a Duraspark and mounting a two barrel carb. You can use an adapter and run Weber but that can cause hood clearance issues and you still have all that flow going through a small hole in the intake. A different route is to get a later, large log head and have it milled flat and enlarge the carb opening and drill and tap an autolite 2 barrel carb spacer (the spacer I used had the voids filled with JB weld). The carb bolts directly to the adaptor. The Autolite 2 barrel is simple, reliable and common and the one barrel cabs can be finnicky. Going beyond this, additional mods ( cam, aluminum heads, roller top end, etc) and you start getting in some big money. Check out my profile- I could have build a supercharged V-8 easily for what I have spent on my six. I do enjoy the uniqueness of my car but if I had to do it all over again, the smart money would have been on the V-8. PS-ignore the advise about putting the exhaust spacer in the center ports- it is a PITA for no gain.