I have a 1976 Mustang II and I need help with figuring out what to go with, header intake carb and other things. I have the 4 cylinder and I know only simple things about upgrading it. Thanks in advance.
If your car is a factory 2-barrel carbureted car, you actually might not want to bother with an intake or a carburetor. The 2-bbl intake from Sheffield installed from the factory actually flows VERY well, and the Holley/Weber/Motorcraft 5200 family of carburetors is a really good design. It's pretty much half of a 4-barrel carb, with progressive venturis. If you live in an area that doesn't check for such things, you can delete the EGR plate between the carburetor and intake with a Mr. Gasket 2-bbl-to-2bbl adapter, then remove the pipe that runs to the exhaust manifold and install a pipe plug in it. You also could upgrade to a Holley 2300 or Autolite/Motorcraft 2100 family 2-bbl if you REALLY wanted to, but you'd have to fabricate a throttle cable bracket.
If your car is a factory 1-barrel carbureted car, then you're desperately going to want to upgrade. Get either an OEM 2-bbl intake, a 2.3L lower intake and the adapter plate from Racer Walsh, or find an Offenhauser 4-bbl intake on Ebay. If you find the 4-bbl intake, the carburetor to go with is the 390-cfm Holley 4-bbl, if you go with one of the 2-bbl intakes, any of the 2-bbl carburetors I mentioned before will work just fine. Heck, at one point I had a 500cfm Rochester 2G carburetor from a Chevy pickup on my 2.3... (long story)
On the exhaust side, the stock manifold is pretty decent for a cast iron piece, but a little junkyarding can get you a big upgrade in the way of a tubular steel header from a 2.3 or 2.5 SOHC Ranger.
The stock head flows "okay" for the displacement of the engine, but a little port work never hurt if you know someone or are brave enough to try it yourself. If you have deep pockets, Esslinger makes full-blown race heads for them, or if you wanna be different, there is also a Volvo DOHC head that bolts to the 2.3 with slight mods (you'll have to re-align the timing belt as a maintenance item after though).
Last, but not least, would be the camshaft. There are roller follower cams from the aforementioned Rangers that are a great upgrade in low-end torque. There are high performance options from Esslinger. Sealed Power used to make a "high efficiency" cam that I ran in my old 2.3 with a 4-bbl in my '74 II when I had it.