What is the current macOS version your iMac?
Also, I downloaded the BC drivers again, and I do not get .7748, but .7866.
Using Mac model: iMacPro1,1.
Model supported in package distribution file at http://swcdn.apple.com/content/downloads/48/61/061-97204/zjcotww2iqibyvy6wbx3q9d50ca4lhig85/061-97204.English.dist.
Distribution 061-97204 supports the following models: iMacPro1,1, iMac19,1, iMac19,2, iMac18,2, iMac18,3.
Making directory iMacPro1.1/BootCamp-061-97204..
Fetching Boot Camp product at URL http://swcdn.apple.com/content/downloads/48/61/061-97204/zjcotww2iqibyvy6wbx3q9d50ca4lhig85/BootCampESD.pkg.
100.0% 695410688 / 695406870 bytes
Expanding flat package...
Extracting Payload...
Extracted to iMacPro1.1/BootCamp-061-97204/WindowsSupport.dmg.
Done.
You should also get .7866. Can you move the current drivers package to a backup directory, and re-download the drivers again?