[924banger]

How much are you wanting to spend on your mustang?
Just remember that if you drop in a V8 or V6 you will have to change a ton of parts. It's not just the ECU, wiring harness, engine, tranny, tranny mounts, and eninge mounts. You could find most of the parts you need through here or a junkyard but that might take a while to acquire.
You could also look into doing an IRS from the 2000 cobra's(can't remember the exact year). That could run upwards of 2grand though.
Paint is going to run you at least 2000 for a full body color change. You could of course drop that price by doing it yourself or taking it to an automotive college.
Also if you go with a bigger engine, your insurance will probably go up. Depending on what newer car you want, your insurance will go up also.
When you add it all up, it would probably be cheaper to buy a newer car. It all comes down to what you want.