i cannot help here on the technical side, but i tried to redo what you describe, here the steps:
#1) using this rom Mad Dog McCree v2.03 board rev.B ROM ( MAME rom maddog.zip ) -> includes two rom files
#2) using WinHEx -> Tools -> File Tools -> Unify -> Bytewise 8Bit
now choose 2nd and then 1st rom and next save as combined
#3)
wrip from aminet produces the result in attached image.
*image removed to free some bytes*