So with the 2 x 170v winding could I still use a tube rectifier? Or is that too little voltage?
Think of the gz34 as two diodes in parallel. You can use 2 to make a bridge rectifier if you want. 170v is about half of what most power tubes in guitar circuits use.
Also make sure your reservoir (first) filter capacitor is around 22uf if you're using the tube rectifier. Larger capacitances right after tube diodes are stressful for the tube.
2k ohms. If it has 4,8,16 ohm output taps you can derive 3.8k or so using the 16 ohm tap for an 8 ohm load
Search for impedance matching or reflected impedance on transformers. If your tubes expect 3500 ohms of plate resistance (see data sheet), and your transformer is 3500 primary with a 16, 8, and 4 ohm secondary, you are fine. Hooking up a 16ohm speaker to the 16 ohm tap will give you 3500 ohms on the primary, which is exactly what the tubes in the example (pair of el34s) need.
What you can do is hook up a 16 ohm speaker on the 8 ohm tap and get 7000ohms on the primary.
This is how you can use the same output transformer for multiple tube types. This is the 6L6 Datasheet, see under fixed bias with screens at 350v, the tubes expect 6600 ohms of plate resistance. 7000ohms on your transformer primary is fine, as you can underload the tube by using too much (higher number) resistance on the primary. So yes, you can get an EL34 transformer, use it like normal with EL34s, or switch to 6L6s and use the 8 ohm tap with a 16 ohm speaker.
Your 6v6s run at 8000 ohms of plate resistance at 250v, so they might not be the most interchangeable tube with EL34s. You would pull more current than the tube is spec'd for even with dropping the impedance on the output selector like the above example, as it is seeing 7k instead of 8k. Probably not the end of the world if you're not cranking it all the time. 6L6 is a closer match as it's a 30 watt tube (GC version) that runs at more normal EL34 voltages (350ish +).