maybe you could get a verticle air purger and install it coming out the supply side of the boiler then installl the pump above it in the verticle position. The pump is eventually going to go out installed that way but not vertically, look at the paperwork that comes with the pump. bring the expansion tank to the point of no pressure change at the air purger and tie in the make up water and you're set to go.
one thing to remember, when you do something cheap it still has to be right because it can and will haunt you and become more of a PIA. good luck