My '67 Valiant 100 Big Block 4 Speed Restoration

Disassembled the engine...first car engine I ever worked on/rebuild. Found four bent push rods. My guess is stuck valves and someone tried to crank it over and they bent. Lots of gunk in the valley area. The heads are 516 ones. What was the problem with these heads, not large enough valves? And no the center dump exhaust manifold is not from a truck/motorhome.