Hello Gabriel,
you have a severe workflow problem from the beginning.
Did you digitize your DAT audio into FCP ? If the quality is quite okay I’d export OMF-files for sound-post. Even with redigitizing you will experience glitches and stuff ’cause I don’t think that your DAT has been locked to any reference when you digitized it.
Tell us how you digitized it: analog or digital input ?
An OMF export might solve all your problems.
Greetings
Uwe