I know what you're talking about. I have five Hackintoshes set up as Mac Pro 3,1. Most are normal, but two are like yours. One shows an iMac icon in the Finder sidebar and another shows a Mac Mini. But everywhere else in the OS that the icons appear, they show as Mac Pros -- even when you click the Finder sidebar icon and Get Info. It's weird.
My solution was to turn off the device display for the system in the Finder sidebar preferences and to turn on display for all my SSDs and HDDs. Out of sight, out of mind. As long as I can see my drives, I don't really need the system icon there, anyway.
Did you try the customization suggested by nobodynose? Did it work?