First step to install OCLP macOS on these iMacs with replacement GPU is to get starting iMac to latest firmware. Best way is to start with original everything in iMac and install High Sierra. Sometimes I get iMac with HDD wiped so no idea what macOS is previously ran and what firmware level on the logic board. So I typically just boot El Capitan (via external USB SATA SSD) guessing the iMac was surely upgaded to >= El Capitan in the past and install High Sierra. Maybe there are other safe paths but this is the safest I know.
If get a broken iMac such as failed GPU, then this process would be harder. Not sure vBIOS flashed Kepler will boot in El Capitan to do the High Sierra upgrade.