I have done this swap and I got in love with how the gears engage. I guess this is because how the better synchros that others talk about.
The ST205 tranny is simply plug and play, on a ST185. The only thing that is different is the bell housing of the transmission, where it matches the engine block. The ST205 engine has an aluminum flange that matches the bell housing to bolt them together. The ST185 engine uses a bigger steel plate to close up the opening, because the engine block doesn't have that flange. So if you are using the ST205 gearbox on the ST185 engine, you will need to use the ST185 plate to close up the back of the flywheel and prevent dirt from getting in the clutch area.
The ratios are pretty much the same. I didn't notice a difference when driving it. The 4th and 5th gears are slightly shorter on the ST205 tranny.
ST185 (non RC) have the E150F tranny.
1: 3.384
2: 1.913
3: 1.333
4: 0.972
5: 0.731
Final Drive: 4.285
ST205 tranny is a E154F:
1: 3.384
2: 1.913
3: 1.258
4: 0.918
5: 0.731
Final Drive: 4.285