Here is apic of trhe rear water port on a tunnel ram.
its on the drivers side, not the passenger side.
I have another pic of a origional 1964 GM 327-375HP fuel injection intake manifold that also has the water port on the driver side specificly, also.
I cannot find it.
Remember this is prior to the emissions era which started in 1966.
Thats when GM started with the hotter engine idle temp design.
to aid emissions. Hotter water temp at idle- generally less idle spark advance
which make the heads and top of the block run hotter at idle.
Thats part of the reason a stock 1970-71 400 SBC has a reputation of running hot.
Truth is all the engines of the 1970 to 1975 era tended to run hot.
Retarded idle ignition timing.
Makes a big difference. And generally makes any engine run hotter at idle.
See that's the difference--- real world experience and real world context.
Some stuff in this worold was done before the internet came along.
I had a original 400 SBC years ago and went thru all this stuff, then.
Try the water port mod, with or without the steam holes. You'll find it works real good on your 400 and on SBC's in general.