If it helps, I have these:
GVP_HC+8scsi_v4-15_27C256.binGVP_HC+8_SCSI_Controller_v4.5_ROM_27256.binPretty sure I've used the first one before and it works fine. I seem to recall the v4.5 second one didn't work, don't remember the detail.
To use with a 512k EPROM, just concatenate the binary image once. Or just write it as is and make sure the MSB address line is tied low. If the MSB address line is left floating, the EPROM might do unpredictable things, even if you have the same image in both upper and lower halves.
