Start with the little stuff first. You said the valley pan was leaking( im guessing Big block). If the valley pan was leaking and you lost a lot of oil, your oil pressure could have went down from the lack of oil. It's rare the valley pan would just start leaking /pouring oil out without a slow leak first. Check the oil and see if it looks milky. If so i would suspect a head gasket blew or the cylinder head cracked. When changing head gaskets , there is no need for new bolts and no need for sealant. Just make sure you have the gaskets on the correct way, not upside down and the correct thickness . Toqure the bolts down in the propper pattern ,I usually increase the foot pounds 3 times, the 3rd time is the final torque. Then recheck the torque after you drive it. A compression test would be next if the oil isn't milky. You should be around 160-180 psi. Hopefully it is'nt any hard parts that are bad. good luck and let us know how it goes
Nick