My Mustang came from the factory with a 351C/FMX. When I bought it there was a thrown rod and a hole in the block. Ran into exactly the same thing you are: 351C's were like unobtanium. This kinda rambles, but I'll try to describe what I went through to make the swap. It sounds worse than it is.
I got a 460/C6 from a late 70's (1978 IIRC) Ford truck (F150 IIRC). I don't think the FMX will bolt onto a 460, and I didn't even try. The FMX weighs a TON compared to the C6.
I bought a "460 swap kit" from Classic Mustang Parts of Oklahoma. I was surprised to find that the frame mounts look identical to the 351C frame mounts. I never put them side-by-side, but I'm betting they are identical, especially since I later learned that the 351C is basically another "version" of the 385 block (i.e. 429/460). I'm betting that you can go to NAPA (or whereever) and buy motor mounts for a 1971 with a 429 and it will bolt right up. Heck, I wouldn't be surprised to find that the 351C rubber mounts are the same as for the 429. (Once again, I didn't think to compare them side-by-side.)
I also bought a used C6 crossmember. When I pulled the FMX crossmember, they ARE exactly the same. I used the original crossmember and got my money back on the other one.
I bought upper and lower radiator hoses for a 1971 Mustang with a 429. Hooked right up to the "original" radiator, which I had recored.
I pulled the oil pan from a mid-70's Mercury station wagon with a 429. Looks like it was born there. I was surprised to find (at a REALLY bad time) that the truck pan is different and won't fit the Mustang frame. Thank goodness my buddy Mike was helping me and kept me on track, because I was ready to roll the whole thing off a bridge when the oil pan went CLUNK.
I bought all the brackets and stuff off a spare 460 motor (with TWO thrown rods) that a friend had laying around. He thought the motor came from a '73 Lincoln, but he wasn't sure. I used the power steering pump that was on that motor (it looks just like the power steering pump on my mid-70's GM cars) and had custom hoses made at the local NAPA store.
I haven't installed any A/C stuff, and I might never. It's a "fair weather" convertible, so who needs A/C anyway?
I trimmed the fan shroud just a little bit to clear the power steering pulley.
The original speedo cable and accelerator cable hooked right up. I had to fiddle with the throttle linkage bracket a little bit, but it wasn't so hard.
I discovered too late that the C6 transmission I have is for a column shift truck, and doesn't have the correct "arm" for a factory floor shift. The correct "arm" is available, but REAL pricey. Apparently there weren't too many of them made. The original shifter was pretty sloppy and loose anyway, so I just swapped in a
Hurst shifter, took out the original worn-out console, and put in a HumpHugger console. It works good, looks good, and has CUP HOLDERS! I never have gotten the shifter adjusted right (I can't get it to go into first gear) but that's just my own laziness. One of these days I'm going to drag 1320stang (Larry) over to my house and get him to work his magic on it.
I had the driveshaft rebuilt and shortened. The transmission output shaft was the same, so I didn't need a new input yoke gizmo (or whatever it's called). Worked perfect.
My speedometer doesn't read correctly, and may never read correctly. But it works!
Since I didn't use a factory distributor, I replaced the resistor wire from the factory tach to the distributor with a "normal" wire. It works perfectly.
I did this about seven years ago, so I probably forgot something.
I hope this helps, and I haven't bored you to tears.
Wayne.