12" Heatercore mounting question

I was wondering, what is the typical difference in temperature at the very top of a case compared to its very low part? I have an Antec 1200 full tower case, my idle system temps being 27C and system full load temps being 31. I have thought of removing the hard drives rack and mount my 2 hdds in the upper cdroms rack and mount the heatercore verically at the front of the case. Another thought i had was mounting it horizontally either on the very top of the case or the very bottom, since my pump is submersed into a cuboid reservoir. Another issue, i have mounted 2 120mm fans in series on the front side of the heatercore, so will i have to cut off my case so that the air pushed through the fins by the fans gets out? since the heatercore in all 3 cases will be in contact with the case, there is no other way for the blown air to escape. I dont want to slaughter me case

Please advice, thanks

You have the perfect case to place a 120mm fan at the back where the 80mm is and place a Chevy Chevette core with shroud and both ends and another 120mm fan in the front. Drop the drives down and intake the air from the front and out the back. Perfect setup.

Kinda like my drawing, but slightly different.

so, if i get this right, i dish the 12" heatercore and purchase the chevete heatercore? ouch . one more thing, will i have to make an air duct as well and drill the top of my case so that the fan on the shroud sucks cold air?

yeah go with the 86/87 chevy core less than 25$.