I have 4x GIGABYTE GV-R928XOC-3GD REV2 Radeon R9 280X 3GB that I have been using mining zcash. Is it just a matter of flashing this rom in the via atiflash that is in the original post? That's it?
Depends, check which memory type you got with each of those GPUs.
Use GPU-Z. If you got Hynix for all of them, flash with the Gigabyte 7970 Hynix ROM posted above.
Do one GPU at a time in case something fails.