I did the 351W swap in my 68 a few years ago. Bear in mind that my whole car is customized so a lot of the modifications I did to make it work are not an issue if you don't care about originality. Also, it might actually be easier to go with a stock v-belt and pulley system.
First of all, I feel this engine was the best mod I've made to the entire car. I can't believe that the weight difference over a 289/302 is great enough to persuade you to look at a different platform. I do know that 69s came stock with a 351W.
I'm also running a C6 which was a freebie and I purchased a 9" for $100 so strength in the rest of the running gear was not an issue. I know that a C4 will live just fine behind a 351 if it's built right. The 8" might be another story depending on how strong the motor is.
I'm running Hedman Long Tube Headers that I ordered for a 69 with a 351 and they didn't fit when bolted up. All long tube headers I've seen have collectors that angle in toward the trans on a Cougar. That's nothing that a porta-power can't fix which is what I did. I also had to dimple the one tube closest to the steering box.
Now, my steering box is 1978 Toyota... not a typo. It is a power box that I made fit by shortening the steering yoke about 2 inches and notched both the frame rail and shock tower for clearance. After this, two of the original three mounting holes lined up. If going with the original power steering, different mods are needed to clear the header tubes, like moving the slave cylinder. I can't offer anything on manual steering.
I'm also running a serpentine belt system since my motor is a 94 and the water pump was set up for that. My power steering pump, and air compressor as well as pulleys and brackets are all 88 T-Bird.
Yes, the 351 is taller and the bracket for the power steering pump and compressor needs to be modified to reach the threaded holes in the cylinder head. I'm also running an Edelbrock Performer RPM Manifold and 4" K&N Air filter. The hood won't close with these items but I was already running a functional cowl induction hood scoop so that wasn't an issue either.
Motor mounts are exactly the same but I did slightly modify the throttle linkage a little to the right side to clear the back of the hi-rise manifold. It was getting hung up.
One last modification was some sort of restraint system on the driver's side of the motor. Yes, the torque is considerable more and I did break my first motor mount. I'm running a short length of chain and a turnbuckle (for tension adjustment) down to the subframe rail. With the serpentine belt accessories in place, this is not even seen but works very well.
I have a picture of my finished motor installation in the 68 gallery.
http://www.mercurycougar.net/gallery/showphoto.php?photo=1160&papass=&sort=1&thecat=502
Sorry for the long post but hope this adds to all the other great advice you've already received.
Mark