1966 mustang help needed?

might want a bigger radiator
make sure the 289 has a provision for the pilot bushing... Goes in the rear of the crankshaft to keep the input shaft of the trans centered
exhaust
but car/frame mods, none needed

It depends if you want to do the job right or just drop in the V8.
The lug pattern is wrong, it takes a different cross member for the transmission, and at least the front springs are different.
So you need to go to a 5 lug pattern, get the right cross member. The rear end needs to be changed over to a 9". I am sure there are other details.
Here is a link to the short list:
http://www.stangerssite.com/conversions.html