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Yes. That seems to be the groups advice. But let me give you details.
Any manufactured amount of pressure is the one to follow..even when their advice is found to be wrong in many areas over time. They make the engines. They do not use them for millions of miles as we do. I have Workshop manuals from the Buick era and it is surprising how their advice changes as they learn from the field.
David, the higher the pressure of the coolant (determined by the pressure cap) the lower the chance of a coolant boiling, assuming everything else is as it should be. On the other hand, the higher the pressure, the more the water pump has to work and more the coolant fitting and connections have to be leak tight. So both lower and higher than 15psi is not advisable, but if absolutely necessary go to 16 psi, not 13psi. Altitude and coolant-to-water mixture are also KEY. Here is a example. (I will find a better one including 15psi and altitude for gomog. https://cdn.shopify.com/s/files/1/0...ressure_Chart_1024x1024.jpg?v=1642460070 You will notice that 4 psi makes a difference in boiling points. In hot Texas you are more like to find boiling over with a too low pressure cap. (13 psi( tan a correct one.)
Excuse my pedantic explanations but after 30 years, I loathe telling people an answer without their understanding what's behind it. It is how I prefer to be treated. We are all merely amateur hobbyists, but not unintelligent. IN SHORT.

1. Put a blanking cap on your rad.

2. Put a pressure cap on your expansion tank, which MUST be higher than the top of the rad. (Trust me) And in fact, it already is, right from the Morgan Works 30 years ago!

3. One other codicil. You are in the USA where it is easy to buy a safety rad pressure cap. In Texas, there are now as common and the same price as the older less safe ones used 30 years ago. The reason is simple. We do not always tighten the caps fully. Additionally Morgan vibration can loosen them. At best, that will lower your pressure and protection. At worst, well....use your imagination. A safety pressure cap for you is a nil extra expenditure or hassle to avoid this. These are caps that have a small lever that can only be shut when the cap is fully seated and make the caps vibration proof. Pick one up on ebay. https://www.ebay.com/itm/155220098426 I chose an expensive polished stainless one for a Mini Copper. I am community-infamous for my weakness for polished metal.

gmg