Well I aint no expert and I dont play one on T.V. niether but looking at the big picture I would start by getting rid of that damper in your stovepipe and unless I'm dizzier today than on other days I'ld say you got a backwards lean to that small horizontal piece of pipe that goes into your thimble.
But I'll admit I was thinking the same thing... I was hoping that an 8" OD liner might squeeze in, but maybe not.