A 2-261 Chevette core will do you pretty well. It is a 6" x 6" x 2" cooling area + tanks & inlet/outlet. You might need to install it laid over on it's side as with the tanks it will be taller than your specs. It is also 1/2" thicker but this will add a good deal to it's cooling.
For a CPU block this core should serve you well.
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