If you have a shot at getting a 426 Hemi to put in it, you should grab it.
The next best thing to numbers matching is a 426. It may actually be worth more with the Hemi, as long as you get the car back to correct as possible in other facets of the restoration.
I just finished a full restoration on my 69 convertible. Mine was probably a little worse than what you're dealing with in terms of a starting point.
Either way, it's difficult to draw the line when restoring a car. Especially a rare converible.
I found myself initially thinking I could restore only certain things, and half-ass the rest. Then you get to a point when you have to restore absolutely everything, and that's an expensive and time consuming endeavor.
Best of luck! I promise that if you restore it, you'll have the time of your life cruising that baby around. The looks I get in my convertible are priceless.
Here is a pic of what I started with in comparison to what it is now. It took 5 long years, a lot of frustration, a loving (and forgiving) wife, and way more $ than I'm comfortable talking about.