Ian,

For a lot of slow 12/8 50s ballads, I have used an old all-MIDI style called SAM_COOK.STY. The most important part of that style is the classic 12/8 "triplet" figure played by the acoustic piano on the string part and the bass line played by the finger electric bass on the bass part. You may want to eliminate (mute or replace) the rather cheesy electric piano playing on the piano part. There may be some decent RealTracks that would fit in well with the MIDI piano and bass tracks to fill out the sound a bit. These could be put on the piano and/or guitar parts.

Andy