it would make sense if the coolant came back out the block, but due the way the block flows by restricting the coolant for the first or second can actually hinder it's ability to cool. Especially at the cool side of the engine, because coolant enters hotside and exits both cool and hotside of block to the head.
I think the best way would be to open the rear of the head for a fourth exit, then have it run to a swirl pot sump, run a connection to the return line to the radiator, and one to the coolant balance tube, where the pump sucks from. This should help flow for #6 and drasticly improve flow for the entire block.
my concern for equal cooling is due to turbo charging application as well as high rpms used after this build.
I believe a combination of ceramic coating the pistons and the combustion chamber heads and block cooling, will ensure an equal burn.