[Mondial]

Yes, you definitely have a short to ground somewhere. Don't measure resistance to R100, measure directly to ground.

If you are reading 0 ohms from the green center tap wire to the chassis, then there probably is a short in the transformer. It looks like you resleeved the transformer leads, so I would check there first. Maybe the green wire got pinched or the wire is touching the core or case inside.

In any case, disconnect the green wire from the terminal strip and measure resistance to chassis ground. Likewise, measure C101 negative to chassis. See which is shorted to the chassis.

I am very surprised that you said the radio played fine, because with the green wire shorted to ground there is no negative bias on the output tubes. In this condition, the audio would be very distorted, which you did not mention.

This also explains why the 300 ohm resistor is getting very hot. With no bias the output tubes will draw excessive current ( unless they are so weak they can't).