Brandon,
I am going to make the same mods to my car. The parts being used will be period correct for 1968. Tri Y headers, FORD 4 bbl carb, I have a 1969 over the counter FORD dealer SHELBY intake. Even though these parts weren't on the car when it was built, they are mods that someone could have made when the car was new. To get the basics about this car stuff go to a book store and get 'Auto Mechanics Fundamentals' by Martin W. and Martin T. Stockel. For more model specific information get a good 1968 shop manual, or the original factory assembly manuals, I got the assembly manuals at a swap meet. Good Luck.
Jay