Any idea if I can use my chimney as a vent? Before you ask, no it's not in use as an actual chimney anymore as I've converted to indirect on demand heat and hot water (saving like 100/month), so the exhaust is now vented through the side of my house.

The interior shape of the chimney and the rising hot air (lower density) causes a one way air current up the chimney. The exaust from a spray booth dosen't work the same. You would be creating a positive pressure situation. If you put a vent pipe up the chimney and a sealed path to the vent from the exaust fan, it should work.

To blow that much air up a chimney you will need to upgrade your fan and seal the chimney around the vent pipe. Save you some money and get a dryer vent hose, make a mount to put it out a window and adapt it to fit your paint booth fan.

I agree, vent out a window. Simpler and cheaper.