V6 bell housing doesn't fit the TS engine. You could use a V6 6-speed box with a 1.9 JTD bell housing fitted, would mean re-shimming the box but its not impossible. A Fiat Coupe 20v turbo flywheel & clutch kit would go in and work. You'd have to get a pair of V6 driveshafts so you can use the inner splines as they're different to the TS driveshafts. They'd need cutting & shutting and balancing by a professional company, I think Propshaft UK can do this sort of stuff. For hydraulics you'd probably need to use a GT 3.2 master & slave cylinder.
As for ratios, 156 2.5, GTV 3.0 and 156 GTA 3.2 all share the same ratios but with different final drives. 2.5 is the shortest geared, 3.0 is the longest geared and 3.2 is in the middle (but closer to the 3.0).