Coolant options

I only have a 367HO with perhaps 400hp. I have no cooling problems...... ever.
I started with straight water.Then as the system proved itself, I switched to the 50/50 common coolant. My junk is ancient. The rad is a 73, the block is a 71, and one of the hoses, the lower one I think is from the 73 also. The pump was new in 1999, and I run a 7psi cap.
But I have a manual trans.
Most of the time, that HO is operating at less than 150 hp, go figure..........