Ford offered a low pressure cap during some of the '46-48 period, from memory I
think 4 pounds. These cars had the overflow above the seal area and so could be pressurized without modification, unlike some years that had overflow where it completely bypassed the cap area.
I ran one of these for a while on my '48, and it was illuminating...the early tanks with large surface area are too flexible for pressure in my opinion. They could be modified with internal bracing, but that would be a serious job. Tanks for later cars that routinely ran pressure were more compact and I would assume heavier gauge metal.
As car warmed up, at about the point at which radiator became too hot for my hand (that would be about 100--110, way below running temp), there would be a sound like "fooomp" and the rear wall would bulge back a bit, sort of hinging where it was soldered to upper plate. As car cooled down, the radiator would pop back.
This was on a good radiator with no leaks in a car that ran right at its thermostat temperature, not at any sort of beyond boiling condition. The radiator handled the load and never popped that seam while I used the pressure cap, but I don't think the long term would have been good.
The caps are still routinely available from regular parts stores and the usual manufacturers...their size is a common truck application still.
