Hmmm, stand off insulators will keep the twin lead away from the poll, or 
shal I say pipe.  Ideally you'd have the center about 40 feet up at the 
minimum, so the twin lead could drop vertically, or close to it.  maybe 20 
feet of mast, but I'd be nervous about going any higher.  Are their any 
power lines that the mast or antenna could fall in to?
Is their any way you can tie the center off the chimney?  That would be 
better.