It's coolant. You know, the green stuff with added corrosion inhibitors designed specifically to be chemically compatible with cast iron blocks, aluminum heads, rubber hoses, copper radiators, etc. You'd probably get away with using steel, copper, stainless, aluminum but NOT the "sch 80 black plastic" approach. Black implies ABS and it is generally used as drain pipe and only rated to 85 deg C [185 F]. CPVC (higher temperature rating than ABS or regular PVC) is only rated for continuous service up to about 200 deg F. I'd avoid the schedule 40 black pipe as well simply because it'll weigh more than twice what it needs to.
some snippage
Another consideration that the coolant may not address is galvanic corrosion. The theory is that some alloys have higher electrochemical potential and, using the coolant as the transfer medium, will steal ions from other metallic components in the system in order to reach a more stable energy state. Theory aside, I'd just electrically ground all metallic components and it'll most closely resemble an OEM installation.
Even CPVC really doesn't go high enough in temp. We melted some on Ken's first engine dyno. Would not even consider trying ABS. If determined to use plastic I'd suggest looking thru McMaster.com's selection. Might find a polymer that will work.
Galvanic Corrosion happens any time two dissimilar metals come in contact with each other. It's a very slow process until water gets involved. then it can speed up rather dramatically.
