I would be at the scrapyard looking for a mint (ish) Modular 5.4L V8, which can easily fit inside what is one of the largest/roomiest engine bays I've worked in.
Seen some wicked, wicked sleepers, and it seems like the guaranteed recipe for making a 2.5 ton car haul, is a 5.4 w the GT40 heads, and a Roots blower sitting in the "V" to cram the motor full of air.
My father was an executive w Ford for most of his career, as either a controller (financial) at one of a few different plants, or attached to one of a number of projects as the senior finance officer or whatever.
He was on the "team" for the last Gen CV/PI, and as much as I distrust American cars in general, I can tell you that the amount of time, sweat, blood, and money that went into making the P71 as reliable as possible without compromising anywhere else, well, I am half convinced that you have to TRY to break the car, and even then (and despite its otherwise complete mediocrity) the 4.6L just keeps rattling around in there!
Do keep in mind that there is almost no single bit if the 4.6 that wasn't already at least a 25-30 year old design feature, and oils have improved absolutely immense amounts more than the basic OHV (pushrod), low-compression, big displacement with low specific output Modular motors.
That said, what they lack in finesse, modern technology, or the like, they more than make up for in being a fantastically fun and inexpensive motor to beat the life out of, then a few hundred dollars and a weekend = a complete longblock replacement.