It is my understanding that any Virus Scan should be installed on the boot drive.
I also use a solid state boot drive and my McAfee is installed there.
Yes Ron is correct. It has to be installed on your system drive, otherwise it wouldn't work properly. If it was designed simply as an on-demand scanner then things would be different but it actively protects.
80gb should be ample for an OS and McAfee with plenty of room left over.