Everything from the bell housing forward stays the same
with the exception of the pilot bearing, which is for a '67 Mustang, and is a bearing instead of a bushing.
You'll need a cross member and a stock transmission mount, an adapter plate, and I highly suggest getting the longer speedo cable. You'll also need the correct speedo gear, and you'll either need to have your driveline modified, or purchase a pre-made driveshaft. It's one easy days work if you have the drive shaft.
Do a search here for "T5 swap", and you should get all kinds of information.
The Ford Six forums will give you all kinds of info on hopping up the six cylinder. When I put the T5z in, the difference was night and day over the three speed. I could get on the freeway a full 10mph faster. My suggestion is wait until after the T5 is in then drive it. If you think you want more power, think seriously about swapping to a V8. It's pretty much even money.
Good luck,
Scott