I read about someone switching to a Ford GT block in a Ford Lightning.
Ford GT engine block is only $2,795 - but it is machined for a dry sump
oil system. It can be remachined for a wet sump.
You may also want to consider a new 5.0L "cammer". It is already complete and you can use aftermarket electronics. The wiring harnesses should be easy to find. I'm pretty sure it is the same block size as the 4.6l 4v just with a stroker set-up.
Our you can find a 4.6L 4V and stroke it to 5.0L yourself. They make big power with Ford Racing FR500 heads/intake/cams and a free flowing exhaust. Probably a lot cheaper than a Ford GT set-up.