Camshaft dowel pin breakage

My son threw a rod. I rebuilt the motor (used the same crankshaft), but not the head. When the rod caps were torqued down, the engine would not turn probably due to transfer material on a crank journal. Backed off the torque one that affected crank journal, it ran but made a rod slap noise. Took the motor back out and replaced the crankshaft, it turned very smoothly by hand. Installed the motor, started it, it shut off and backfired when I continued to crank it over. Upon removing the valve cover I found that it had broken the exhaust side camshaft dowel pin and jumped time, so I replaced the camshaft. It appeared that the valves were not right so I removed the head for further examination. I found four bent valves, so I replaced them all. Put the top end back together, started it up again. It started, shut off. Removed the valve cover, broke pin again. Stumped!
Is it something that I am doing wrong?