stripped cab then repairs and epoxy primer questions

[my kids toy]

I have my cab sandblasted and Now I need to do a few repairs like smooth out the fire wall and a few other holes

What is the best way to approach this ?
I am using eastwoods epoxy primer
I am using Vintage air so I am going to fill in the fire wall before epoxy primer.
Should I do body filler before the primer or wait till after the primer ?
What about other holes that need filled, if I wait to long then the cab will start to rust up again and I do not have the time to fix all repairs immediately.
Can epoxy primer be ground down for welding and then epoxy reapplied ?
I only have 3 spots that need rust repair with no replacement panels only spot repairs.
I also need to fill the gas tank hole and I do not have the time to get to that right away.

Should I shoot it with a regular primer within the 5 days so I do not have to sand down the epoxy primer ? if so what should I use ?