I have an old GM Delco-heat system in the home I just bought. It is a complete system but takes up too much space in my very small basement. It is no longer needed and I was wondering if there is any value to the boiler and holding tank, along with the three remaining radiators in the house?

No value unless to a collector of odd ball stuff. Energy efficiency is the name of the game today and that unit was a real energy hog. I am old enough to remember working on plenty of those systems when they were not that old and I will have to say they were very well built but large is space usage.