Hello and welcome, The world of classic Mustangs can always use some new members.
That said I've done a few 6-8 conversions on the early cars. To keep it simple I'll give a small list for a 66. Doing a conversion like this is completely doable, the problem often encountered is that the price of the assorted parts starts to add up and before you know you've invested more than the car is worth into it. I always try to avoid that and suggest others do the same.
ok starting at the front suspension. You need v8 spindles and steering linkage(inner/outer tie rods,center link,pitman arm,idler arm),might as well throw discs brakes on right $ ? NOT REQUIRED:figure in better shocks,springs, a larger sway bar,and an export brace.
Engine compartment you need a v8 radiator, a v8 engine with full trim and wiring,fuel system, accessories (pullies,fan,starter motor,brackets,etc.), motor mounts, gas pedal linkage(different for 6 and v8),exhaust system.
Then you need the trans,bellhousing,clutch fork,clutch kit,Flywheel,engine spacer plate, shifter,reverse light switch ,cross member (unique for a T5 conversion),speedo cable.
Because you would be doing a T5 conversion you would need to work on or replace the driveshaft, figure $200- $300 for a new shaft from a repo dealer.
Lastly the rear itself, you would want to go with something such as an 8 or 9 inch , price depends on what you want as far as locking rear,what ratio,etc,don't forget rear brakes. While you're at it you might want to replace the leaf springs and shocks.
And of course don't forget you're going from 4 lug to 5 lug rims, so you need a set of rims and a spare.
That was my brief overview list,But it should give you a general idea. Unfortunately when you start pulling stuff apart on old cars you find lot's more stuff to fix and replace,these things tend to snowball. Old car guys call this "While I'm at it,Itis". Because you start of to replace 1 item and end up replacing 5 others "while you're at it" . Lol
Hopefully I didn't scare you off, If you decide to proceed on this car and you need some pre-purchase advice,etc. Let me know.Good luck.
If I had the money I would keep my current car so I'll probably just buy a rolling chassis and build my way up