cooling my slant 6

On an old slanty I would go the other way.

My 360 has run 15 years on a 7pound cap. Still has all the original hoses I installed in 99. And some of those, came out of the recycle bin. Only the bypass was new silicon, and new cheap heater hoses.
If I can get away with that on a 400plus hp engine, I imagine the slanty can too.I run a 7# cap on my 80 Volare/6 as well. 30 to 45 year old rads will thank you for it. And so will that short little nipple under the stat housing, that you have to remove the pump,to replace.

Yeah I know all about pressure and boiling points.But heres the thing ; Modern antifreeze/coolant are not very good at cooling. They are good at two things only; not freezing, and not wrecking aluminum stuff of which there is a preponderance of, on modern engines.To successfully run that stuff,at the modern elevated temperatures, and not boil it requires those, hose busting, gasket blowing, rad compromising, big pressures
I run the best applicable coolant there is; water. With a bit of additives in there for the pump seal, and the Eddies.Since the coolant actually does the job, high pressure isnt needed.
Theres only one downside. It has to come out before winter.Thats a very small inconvenience. I just reach in there and open the taps.