I suggest you read again.
The DVD doesn't fill up to 4.3 GBs to maintain quality.
Encoding is part art and part science.
If the 90 minute preset yields a size the leaves extra space on the DVD, it is for a reason.
A bunch of us have given you the workflow.
Exactly what else do you need?
Encode the 25 minute NTSC DV export using the 90 minute preset and use the ac3 audio file.
The fact that it doesn't "fill up" the DVD is not relevant at all.
If you spend some time reading here and elsewhere, then that fact becomes readily apparent.
You ask for advice.
People give it.
When it apparent that you do not know the process and people observe that, then you become indignant.
C'mon, do some reading, and take the multiple suggestions offered here and give it try and then come back with questions