The MG engine is fairly similar to the Volvo. Here is a link to the potential causes of dieseling in our MG engines. Please note that the combustion chamber is in the head, not the block.
https://mgaguru.com/mgtech/engine/ro101.htm
By running with the vacuum disconnected, you are getting no vacuum advance from the distributor, only mechanical advance. Consequently you are probably running too retarded. This will contribute to overheating and perhaps your other issues.