The drives are just regular TRS drives i believe....the 390 i'm not sure about. The only numbers i'm aware of were the 330, 370 and 420 combo's and most times it stood for the HP of the engine in the boat....hence if you had a 330 TRS it was a 330HP engine in there.
The TRS drives can take about 500-600HP stock if you dont beat on them too hard. They can be built to take more if need be. Contact Marc at BAM marine or Sterndrivespec on the board here....i believe they do alot of work on these drive....i had mine done by Marc and lasted 3yrs of abuse behind some 500hp engines