If using vented cap, one nipple on the bottom or one nipple on top as long as there is an internal hose that will allow suction of coolant from the bottom of the tank, internally.
If using non vented cap, make sure you have two nipple, one on top to vent out overflow of fluid. The other nipple can be also on top if there is an internal hose that will allow suction of coolant from the bottom of the tank, internally or it will have to be in the bottom so that it 8will* get every last drop out of the tank.
The factory tank has two nipple on top, one for over flow, one for receiveing the hose from teh thermostat neck, and it has an internal hose (that always like to come off) to allso suction of coolant from the bottom of the tank.
In the past, I've come accross many DSMers that use a one nipple tank on the top with no vented overflow cap on the tank. This is BAD. When coolant pressure rises up and the coolant cap pop to allow excess system pressure to flow into the overflow, with a non vented overflow cap, the over flow bottle will also fill up with pressure and when pressure is equalized, there will be no flow going into the tank anymore, and the system will not be able to alleviate excess pressure in the system. Plus with the no internal hose with a top mounted nipple, coolant will flow in but will not flow out anymore. During cooldown, coolant stay in the overflow and the system will develop vacuum so bad that the coolant hose will collapse. Seen this before haven't you?
Remember that coolant will be pushed out towards the tank when motor is running (the cap regulates pressure of the cooling system, anything extra will be vented out towards the reservoir) and it will need to suck in coolant back into the system from the reservoir tank during cool down. Meanwhile the overflow tank will have to keep pressure in check so itself doesn't become pressurized.
