Have you got the amp to a point where it works, as in plugging in a guitar will pass a signal to the speaker? Sometimes, this screeching sound is when the negative feedback loop is connected incorrectly and it actually turns into positive feedback. Try reversing the leads from pin 3 (plates) of the two 6L6s that go to the output transformer.
Yes, you can connect pin 8 directly to the chassis next to the tube socket. You might want to insert a precision(1-5% tolerance) 1 ohm, 1/2 to 1 watt resistor between pin 8 and ground. This way, you can use it for sensing current for biasing purposes. You use your meter's millivolt range to measure the voltage across the resistor and it translates directly to amps since it's 1 ohm. So if you read 40mV, it's cathode current is 40mA. This is the safest way to set bias because you are measuring less than a volt. If you are going to change it to adjustable bias in the future, then they are already in place. The 1 ohm value will not change the performance of the amp. Even if you don't change it to adjustable bias, you can still use the resistors to monitor bias current to see if your tubes are closely matched.
From what I can tell, an original 5F4 has a bright cap across one of the volume pots, so the amp should have the high/low, bright/normal scheme. It's the top channel that has the .0001 cap across the volume.