The back of the water pump will look like the pic below (or something close depending on the exact GM application.)
Cooled water goes out the two mounting ears into each side of the block, then up through the water passages where the block and heads mate, and then up into the intake manifold and out the front. I'm not sure what the temperature differential would be between water in the block around the cylinders and water coming out of the manifolds. The "fire" is in the combustion chambers so there is certainly more heat being generated at the top of the engine.
You may be able to design/build your own heater core and blower but I think you would need a stronger fan than a computer fan. The stock fan is a centrifugal one. Maybe you are a whiz with building things like this but for my money and time I would just go with the ready-to-install one from Heatercraft. http://www.heatercraft.com/heaters/
"For every complex problem there is an answer that is clear, simple, and wrong." (Henry Louis Mencken)
2005 X2, Viper Red, MCX, Acme 1285, PPass, rear 750 sacs, KGB, IBS, Bennett Wake Plate
(previous) 2001 X5, 1991 TriStar 190