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I did this swap on my 65 stang.. what a mess it was (Though I went with a newer model 302)! I've heard a many a people say that its cheaper to buy a V8 car than to swap to one. I don't think this is necessarily true if you can do most of the work yourself and you go junkyard hunting.
Since you're going with an older block then this should make it easier in a few minimal senses.
Now since you're doing a 68, I don't know if the steering components/brakes are different. Those should be changed to V8 standards if they're not there all ready.
For the C4 you'll need a new bellhousing and blockplate for a V8 (I'd think?)
Probably a new radiator, motor mounts, could probably use your existing wiring harness, but some wires might need to be lengthened. A starter to bolt up to the new bell. Accelerator pedal linkage could be different too.
Your driveshaft will most likely be the wrong length now as well.
I can't answer any of these exactly since I don't know much about that year model. Maybe someone else can come along and give you a better idea. And maybe the above might help to an extent.