i have a 351m in my 1989 f350. ever since i got it which is about 6months ago. wen i take her out she runs hot. i start her up go play around in the mudd and cruise and stuff and then about 10 - 15 mins later shes hot. i changed the thermostat to a 180 degree but still ran hot. i believe the water pump work because i have watched the water in the radiator being pumped. i rly want to get this fixed so i can ran her more than just 10 mintues if anyone can help i would greatly apperciate it! old thermostat had the guts taken out of it

 

I would start with having the radiator flow tested. Have it checked for head gasgets leaking as well. Internal engine damage can cause overheat as well.