Everytime I try to add my new fftpack (I've made sure it's the exact same size of the original), It never let's me either,
A. Patch afterward with FFTPatcher
OR
B. Let's me boot it up.
It seems UMDgen like to resize it to 379mb from 399mb after saving it.
what do i do?
That's gonna happen as far as resizing goes also after adding the FFTPack you will not be able to patch so your problem makes sense.
To get around it patch the ISO before adding the new FFTPack. Problems will occur though, it is inevitable.
If there is a way to fix the problems it is probably best to ask one of the project leaders.