cpu block that I can make from copper pipe
 
Currious not saying it can not be done I have never seen a cpu block made from copper pipe or I am missing something?
 
Do you have the heater core laying around and is it made from copper or aluminum? If you have to purchase the core from a junk yard or as a new part I would ask myself these questions. If the heater core is in very good condition and made from copper ask these questions.
How old is the core?
Are the fins on the core painted? 
Is the core dirty on the inside?
What will it cost to vat the core for cleaning and do a pressure check to insure there is not any leaks in the core?
How many water blocks are in the loop?
 
Copper heater cores where used in water cooling loops a number of years ago. Today a lot of the heater cores and radiators are made from aluminum to lower the cost of mfg. Solid cooper heater cores and radiators are available very expensive. I would be concerned with the medals used in the brazing process for assembly. Do not see a reason to use a heater core or a radiator for a car  when there is a number of very well made radiators for liquid cooling a computer unless you are wanting to do a nostalgic build that's just saying? Alphacool made some of the first radiators for computer liquid cooling.
 
Radiators prefer to use for fan RPMs between 800 RPM thru 1200 RPM the radiators are designed for liquid cooled computers. These radiators have excellent flow rates and cooling capability including build quality.
 
Alphacool NexXxoS Monsta
XSPC V3 only
Black Ice SR-1 
 
 
 
 

Remember You Do Not want to Mix Copper and Aluminum in the Same Loop.
This is a good site to review about Computer Water Cooling.