Heh... not going to happen, bud. 40C at full load, even on a 2500K (but at 4.8 GHz and 1.5V) sounds impossible or your load was something jenky like gaming and not a proper stress test.
I stress tested it with prime95 and running the gpu full bore. I also was running boinc and folding@home 24/7 then, at least not when not using the computer. 40c max load was what I got.
The same cooling loop with only a 3930k couldn't keep that cpu below 80c when overclocked to 4.6ghz.
I had a monsta 240mm long 80mm thick radiator with 32mm panaflows and 25mm spacers between the fans and the radiator. The fans ran at full speed. Swiftech waterpump and pump with an alphacool full cover gpu waterblock. The radiator is so thick that I used it as the resivor. I have pictures somewhere. I bet I even made a thread about it back then around 2013.
A 3900x doesn't use the amount of power as a 3930k, but does have a smaller node. Since I am running it at stock, and letting it overclock itself I should get some great temps.