Heya Alexander,
You can probably enlarge the opening in the frame where the trucks sit to allow them greater swivel (you don't have grind the tank, actually)..
But then you will run into other issues-- The driveshaft and universal joints, when spinning, tends to exert forces on the truck to force it to straighten. Also, the trucks might bump into things on the shell like jacking pads that prevents them from swiveling any further. So those are a few potential issues you may have to deal with as well in addition to the frame clearance.
Anyway, these days my 6-axle locos don't get much use... 4-axle power work much better on my tiny layout with its 18" curves, so these days the vast majority of the locos on my layout are 4-axlers... Geeps, GE Dash-8 B's, Geeps, U-boat-B's, Geeps, Alco S-series switchers, Geeps, EMD SWs, Geeps, Amtrak F40s and Gennies, and that's right, more Geeps. Did I mention Geeps? sign1