72 318 with 360 heads, marks in pistons

I am in the process of rebuilding a 76 318 mild street motor for my 65 dodge dart. Saw on craigslist a 72 318, 360 heads,holly street dominator intake with matching valvecovers and a stock 904 trans in a 2wd dodge truck.$300 running. Really wanted the big valve heads and expensive intake so he delivered it to me. Here is the catch(found out after he left) low oil pressure(20psi)when hot.took heads and intake off to inspect. Looks like heads just worked on but the pistons appear to have valve marks in them like they were hit.valves and springs in the heads look new. What would have caused this and would it affect oil pressure? Thanx in advance.