I need major help

The forum search engine does not do terms like "I6 to V8", so there's sticky thread up here for that. Do those things first (upgrade to V8 specs on drivetrain/suspension/brakes) and the fitting of a late-model engine becomes much simpler. Have you considered using the newer trans too?

2

the c4 you have now will work, you just need a v8 bellhousing.

Like rusty said, swaps are readily available, it just depends on how much you feel comfortable with doing yourself. I saw a great simple one in the mustangs plus catalog awhile back. They used the stock type setup out of a donor car, which is what I imagine you would be doing.

other than that, you need motor mounts and off the top of my head, you need to get a v8 throttle linkage/rod or modify your 6 cylinder one. Originally I cut up my 6 cylinder one (shortened the length across the firewall, raised the height of the pin end) but I found a v8 one for cheap that I switched out for.

Can you survive with that 6 cylinder c4? yes. Can you survive with the 6 3/4 rear end? yes. I had a junk 302 that made roughly 220 to the tires that ran around on the street fine with that stuff. Took it to the track once on ET Streets and the trans starting slipping.