I'd ask which of these two trucks would be better for you, and focus on that one.
Unfortunately I am FAR from Missouri so I can't lend a hand.
You say you were driving the 300, and had it working.
What parts were stripped?
What other damage did the thieves do?
I'm just grasping, because around here it usually one extreme or the other.
Joyriders, or they find the stripped hulk burned to the ground.
The 351 may just have a head gasket in backward.
I've seen that too many times on Windsor's.
But whatever is wrong is going to take some diagnostics.
Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with a Holley 80508 and Performer intake.
Too much other stuff to mention.