I had -6 feed straight from each side of the pump to the top of the heads between the center cyls and an -8 out the side to an expansion tank. The tank just fed back into radiator.
We fed into the sides forever but once we started making good power (1800-ish) the heads were going soft in the center, would cut copper shims and stack them on the head gasket to go rounds. Pulling these heads are a nightmare, I hated it. Finally figured out the steam wasn’t letting the water get in there from the sides, so we let it do what it wanted to do and vent. never had a gasket issue again. This was also on a conventional block so we had to plug most of the sb2 improvised deck cooling passages, may not have this issue if you use a sb2 block, hard to say.