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 Many here suggest the policy of figuring out how much it is going to cost for EVERYTHING you need then doubling that figure
This question has been answered HUNDRED of times here. Mustang Monthly and other publications have done excellent illustrated articles. I suggest that you read them carefully.
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.