Gotta second the warning on cheap eBay turbos. One legendary horror story was when a turbo from SS Autochrome self-destructed, and the buyer then realized the compressor housing had been cast in two pieces and then epoxied together.
So, make sure it's from a reputable manufacturer - Garrett, Turbonetics, or at least Master Power. The turbos from Communist China that turn up often on eBay have a bad reputation, and it's deserved.
For most slant six projects, though, I'd recommend something else. Both the GT45 and T70 are really, really big turbos. Garrett lists the intended horsepower range for their GT45 as 700 to 1,100 horsepower. If you're planning a project where one of these is appropriate, it definitely won't be cheap.