How much?

Welcome to the boards. I would try a search with the word "swap" but I think most of our archives have been lost.

You can use the engine, trans, harness, comp, seats right off the bat. This isn't a weekend undertaking, and I don't know your skills but that should give you an idea.

You have to be careful when putting a v8 into a 6 cyl classic because the suspension, steering and braking components aren't up to par. You can certainly drive the car with the 6 cyl stuff still on it, but I wouldn't recommend it.

Here's some helpful links that I found when I swapped in my t5.
http://www.desertmustangs.com/Write-ups.htm

http://www.geocities.com/stangnetswapper/

I think there's a fair bit that you can put into the convertable. But I'm not the guy to ask.
I suggest that you start a new post in the tec section and title it something like "need help swaping parts from 5.0 to classic" You get what I mean.
There are quite a few guys on here that have done what you are about to do. They will have some great advice for you and I'm sure some great links where to get additional parts. Also a great way to get all the help you might need is to include pictures now, and every step of the way. Take a look a the SN65 thread it's amazing and a great way to get help, advice and opinions. What more could you ask for?

BTW, welcome! I can't wait to see the pictures and progress!