Putting a 289 or 302 into a 1975-1978 Mustang II is a bolt in affair, and a 351W is almost the same.
Putting any V8 into a 1974 Mustang II is different.
You've got several issues you will encounter while trying to put a V8 into a '74.
1. The radiator core support is further back: You can mount the radiator on the other side of it and use a pusher-style electric fan for the room, but forget about A/C; you're NOT going to fit a condensor in there anymore.
2. Motor Mounts: the 1975-1978 Mustang II motor mounts and frame mounts bolt into a '74, but they are hard to find, prices start at about $175 for a used complete set on Ebay.
3. Transmission crossmember: The 2.8 and 2.3 cars had some differences to the transmission crossmember, you'll be best off getting a V8 crossmember, and a dual-hump one if you can find it.
4. Rear-end: Many 2.3L cars came with the 6.25" rear end, which just isn't adequate for even a stock smog-era 302. Find an 8" Mustang II unit, or a 9" Lincoln Versailles unit.
5. Transmission: You've already found a C4, that's a great start, but it won't easily fit, especially with headers, without using the Mustang II specific bellhousing and an appropriately-sized torque converter.
6. Oil Pan: You'll have to make an oil pan fit, or find an oil pan from a Mexican 351W-powered Mustang II (while we were stuck with 130 horsepower choked-down 302s at BEST, in Mexico the Mustang II could be had with a 302 in 1974, with a four-barrel carburetor, and even a 351 Windsor some years!)
7. Driveshaft: You'll need to find the right one for a V8 with your chosen transmission.
8. Wiring: Being a '74 is actually a blessing here. The car is already wired for points-type ignition, so you can easily just buy a rebuilt points-type distributor at any parts store, swap in a Pertronix module, and use the factory wiring. The only thing you'll have to worry about is changing out the tach module inside the instrument cluster if you plan to use the factory tach.
9. General clearance issues: A sledgehammer and a grinder will become your best friends if you do this swap on a '74.
10. Structural Issues: Last, but not least, you've got to consider that this car wasn't really meant to have a V8. Ford changed more than just motor mounts for 1975 to make the return of the 302 happen, there are significant changes to the structure of the car. At the very least, you'll want to invest in a set of subframe connectors and V8 front coil springs.
I researched everything I could find on this swap back when I had my '74. In the end, I decided not to do it (I'd already done it on a couple of fox-bodies, which are much easier) and sold the car after I bought my '76 with it's factory V8. If you've got your heart set on your '74 (Lord knows they look SO much better, that's the one thing I miss about my '74), and you know you want to go for it, then by all means, do it!
1974 Mustang II Mach I 351w Swap. I have questions. 
