I haven't done the Bird swaps of 6's to 8's, although Im in the process of swapping one 8 into another car, etc... But Ive known others who have swapped Mustang 6's for 8's, and Id fathom its probably similiar.

You're likely looking at: suspension parts (heavier springs, etc.), mounts, fuel lines, trans?, driveshaft (might be different length). Plus misc hoses, and some electrical wiring that will probably be slightly different. Perhaps shifting linkages as well? Your differential might need swapping out as well depending on how big a change you're doing.

Hopefully someone here who has done it before can give a little better specific detail, but thats the stuff off the top of my head. If Im wrong on any area, Im sure the gurus will correct me..

btw: If you need a bunch of those V8 parts to make that project happen, let me know as Ive got two donor cars Ill be putting up for sale soon.