I’ve got a freshly rebuilt 6 cylinder 2.2 in my car with a new set of PMO Carbs, I’ve had the car set up several times on a rolling road and it runs real well, the trouble is after 200 miles odd the spark plugs on cylinders 4&5 soot up and don’t fire properly, would fitting hotter plugs on all cylinders cure this and would it damage the engine at all
Thanks

Firstly DON'T fit hotter plugs, air-cooled engines are never happy with this,besides its just a bandage for another issue which I suspect is in your ignition.
CDI units were fitted by the factory in the late 60's to stop the plug fouling issues and this cured all of the earlier motors issues.
You should have a 3pin Bosch CDI with stock high voltage transformer style coil,if all is stock you will be running points....what do you have ?.
Failure of these old stock Bosch units is fairly common now as the are old tech and just old .
The issue you have is that a dyno will read your output for max HP @ a higher RPM that you usually drive in so most likely you are fouling plugs using normal driving range RPM … make sense ?.
I spent a huge amount of time ($$$) setting up the system on my 2.2t motor with the right stuff and the end result was expensive but well worth it....happy to share,cheers.

As mentioned, fitting hotter plugs may not be a good idea. But first things first - what plugs are you running?

Fouling can happen for many reasons, but it's usually one of these:
Internal engine issue can foul plugs with oil - hopefully not in your case since it sounds like you just had it rebuilt
Ignition issue - CD, wires, plug, coil, etc.
Fuel issue - carbs running too rich

Your other plugs look good? If that's the case, it could suggest wires/plugs on the cylinders that are fouling, or a localized fueling issue.