What would make both stock 5.0 exhaust manifolds to crack?
I checked out a nice 93 F150 5.0 4X4 for sale today and both exhaust manifolds were cracked (and jury rig patched with something that looked like putty) between the ports for the center 2 cylinders and the port for the rear cylinder.
I've never seen cracks before on both manifolds.
BTW,this is a 49 state emissions truck.
What would a good set of used manifolds run me? Maybe somebody has a good set of take offs laying around.

 

My son had a 92 with the 5.0 and when we took off the passenger side manifold to install headers it just fell to pieces in our hands. Dunno why. Maybe poor quality casts or to many hot/cold cycles or suddenly drenched in water while hot... Dorman make replacements or go with shortie headers. About $70 ea with hardware for the manifolds and $180+ for the shortie headers.

 

Bad motor/tranny mounts can contribute to the cracking, might want to take a look at those also.
EDIT: take a look at all the exhaust hangers as well. weird twisting angles generally causes cracks. I'm not ruling out bad castings, but I would definitely look deeper.