You didn't say how much the engine from the "burned truck" was damaged by the fire.
Maybe not at all? I did this a few years ago when I got a 5. 0 liter Ford for $75 from a burned Expedition. The fire was so intense it got into the cam drive cover and melted the front cam bearing and the front main bearing. The valve cover gaskets were also toast, making the valve springs suspect (I changed them). I ended up stripping the thing down to the bare block with just the pistons and rods left and cleaning it thoroughly. Good thing as there was a lot of bearing material in the oil passages.
It ended up being a good deal, but was a lot of work. My point is; be wary of the $100 V10. It might be a screaming good deal or a real PITA.