Can a 1 1/2' pipe be used to vent a 2" shower drain?

Im looking to put a Miller trailer furnace in my garage, and I have a 275 gal vertical tank to use, I am curious if you guys can walk me through the install, I know very little about plumbing or the termanology so please bear with me. I understand the tank has 4 bungs in the top, one for the fill pipe (2") the one at the oppisite end of the tank for vent & alarm (2") on for the fitting that the supply and return (3/8 od copper) lines go through, and one extra. I dont know the name of the brass fittings that go into the 2" "double tap" fitting, i know it uses 3/8 od copper, so should the holes in the 2" fitting be 1/2" pipe thread so the copper will fit though it, also I will need a firematic valve for the supply line, and it was reccomended for one of the tank drain holes, and to plug the other drain hole, I was told not to put a firematic valve on the return line, to use a ball valve (so you can change the oil filter?) on that line. Any help would be appreciated. I was also told to have the fill and vent lines close together and the vent line 4-6" higher than the fill line( after they have exited the building) that this will aid in filling so the guy can hear the whistle better.