6cyl to 8 cyl - Where to start

OK, firstly try doing a search because I'm pretty sure you're not the first person in history to ask this question.

But briefly here's what you do. Put the car up on stands and rip out the entire suspension. Replace with V8 gear. Virtually nothing is the same, especially the steering gear. Off the top of my head you might only be able to salvage the lower control arm and radius rods and the upper arms although I'd suspect with their age they'd need replacing or rebushing anyway.

I would have thought that an I6 fastback would be a bit of a rarity, maybe its better off being left that way?

Don't know about the spring question, but generally a shorter heavier spring along with the 1" bolthole drop gets the right height. Once again, searching should get your question answered.

the 620 springs will give you the right height only if they are shortened. mine were from the factory, and didnt even need compressed to install. with the shelby drop, you are going to be about 1 1/4 inch lower than your stang was from the factory ( i say this because your car might have settled a bit over the years. ) also, the v8 spindles will bolt up to the 6 cylinder suspension, if you choose to do the conversion that way... my car had the 5 lug spindles on it when i bought it, but the tierods, etc... were all 6 banger.