Alwyn,
I had that guitar once, but it had a different tremolo on it and I got to say that the pups were a little bit of an acquired taste. For clean work - I'm talking positions 2, 4 and 5 these pups are excellent. However if I used gain then they didn't really cut it for me. You could do a lot worse than installing a JB into the bridge and perhaps keeping it as that. I dont know if it makes much difference but my 812S has the same pup config as your 612 and I have no issues with sustain at all. However, these are
stock custom DiMarzios....
On all of my pacificas I have changed the pups, this is my only critisism of them.
As for the sustain, I cant say I've ever really had a problem with it.....only the tone.
Take care and keep us posted on your progress...
Chris