It has to be in a separate "compartment" to the cabin. In the case of the KE70, you will need to seal off the boot from the cabin. I'm not familiar with KE70's so I'm guessing here.
Can you fold the rear seats down in a KE70? Basically you'll have to get a piece of aluminium or steel sheet, and make a "firewall" between the cabin and boot. The boot is naturally already ventilated normally, through drainage holes in the floor.
If you are going to get a surge tank custom made anyway, I'd suggest going one under the floor anyway, just to keep things neat, and so that you can actually use your boot space. In my car, I don't put anything in the boot because the thought of things flying around my boot and knocking my fuel pumps/surge tank over is no fun. And being a KE70, you can just get one specially made for an AE86 (Anthony Kellam sells these), AE86 ones go under the floor because the "boot" is the same compartment as the cabin.