^^ The Teksid aluminum block is every bit as strong as a cast iron, but sheds about 60 lbs off the front of the car.
I like the GT500 swap idea, and that's a killer price on the engine especially considering that 03/04 Cobra engines are going for about that much, but if $8K is your budget (period) then I'd say that swap is a bit out of reach at the moment.
You would at a minimum need of course the engine, pcm and main wiring harness, I doubt that your existing K member could be used, about the only thing from your existing fuel system that could be reused is the tank itself, that engine would eat your T45 trans
for lunch and probably the rear axles too.... And that's just the few things that come to mind for a guy like me with no experience at all with this kind of thing, I'm sure somebody who knows their sh!t could elaborate in further detail.
Bottom line is it's anything BUT a plug n' play or otherwise direct swap.
If you're really bent on it though, especially considering the price it might not be a bad idea to jump on that deal and just hang onto the engine for a couple years or whatever while you do your homework and round up the other necessary items needed.