matagorda :
O.K., I have not modified cooling system, I have reversed air flow and cleaned, and have not yet fried anything, and one is copper, the others are aluminum.
When you reversed air flow, before overheat or after - this actually mean modifying Dell Optiplex cooling system. Unless you are ready to fry motherboard transistors you better turn you fans again. CPU fan must take air from outside and blow it inside, through the fins. There is a line of transistors called Mosfets, located between CPU socket and CPU fan, when air comes from outside into the case, Mosfets got cooled first, they are as important part of motherboard as CPU! If you reversed air, so CPU cooler fan working as exhaust, those poor Mosfets got hot air from processor, so they will fry soon, no joke!
Now, how PC with copper doing compare to aluminum? Same or much better? Also, have you removed any heatsinks already when you tried to mount retail ones? If yes, have you check how good the old thermal paste was, have you applied new one? And is it was good one, like Arctic Silver?
Now regarding CPU cooler. No, there is nothing out there to mount on those holes. What you can do is to buy extra fans and stick them inside to increase air flow (but air must move the proper way, from outside into CPU and next exhaust through the back panel).
I have very good information about this at home, I will find it and post, latest tomorrow. For now reverse airflow! You are cooking your Mosfets!