68 Mustang

Like anything else...time and money :D

Your best bet would be to find a wrecked donor car since there are DOZENS of small parts you're not thinking of like u-bolts that will eat your wallet alive if you have to source each part by itself.

Not counting the motor/tranny/rear-end here's a quick (incomplete) rundown of things that are different:
Radiator
radiator support
hoses
accessories (alt / ps pump / water pump / etc)
Upper and lower control arms
spindles
sway bar
springs
motor mounts
crossmember (?)
Flywheel
starter
transmission mount
driveshaft
u-joints
leafs
shock plates
rear brakes
u-bolts
wheels (4 vs 5 bolt)

There will be many more too that you will find. You might want to check out the sticky under the "talk" section for converting a 6 to an 8, most of that will apply but going to a Big Block will make parts harder to find and cost you more.

NOTE: I'm not suggesting you not do it, just make sure you know what you are getting into :D Many here suggest the policy of figuring out how much it is going to cost for EVERYTHING you need then doubling that figure :(
 
Since you don't already own the 6, I suggest waiting for an 8. It would be a lot easier to replace a small block v8 than the six. I bought a 67 with a 6 and 3 speed a few years ago, like you, I wanted to put in a 390 that I had in storage. After driving it for a few months, I decided to keep the six the way it is, and I might buy a V8 model if one comes up for sell, pull the 302 and drop in the bad boy.