I did a horizontal mount core in a tight case and added bleeder fittings to the tanks so I didn't need a reservoir.
Check it out here. The core was mounted at a slight angle to help bleed the air out.
I cut a 1/8" NPT brass pipe coupling in half and soldered the two pieces over holes drilled in the tanks opposite the inlet/outlet. I removed both pipe plugs for initial fill and could actually remove the suction side and top it off with the pump on. Having the PC on or off while filling is a personal choice though...LOL.
It was my first water cooled setup and performed pretty well.