If your ST-70 power transformer doesn't hum, buzz, vibrate or run hot AND it provides the proper voltages then it is probably still OK. You are correct in mentioning about tube choices. You can use EL34, 6L6, KT66 or KT77 output tubes with a stock Dynaco power transformer. With an upgraded power transformer you can also try KT88 or 6550 tubes.
If all seems good you won't really get any better sound quality with a new power transformer. That said, if you do use this amp on a daily basis understand that your amp's power transformer IS 30 to 50 years and may have thousands of hours of use. Just be alert for any signs of stress that *could* arise in the future.
To check an ST-70 PA-060 power transformer pull the rectifier tube and leave all the other tubes in there and check some voltages ..
1. Check ACROSS pins 2 and 8 on the rectifier tube socket and look for about 5 volts AC
2. Check pin 4 to chassis ground on the rectifier tube socket and look for 355 - 380 volts AC
3. Check pin 6 to chassis ground on the rectifier tube socket and look for 355 - 380 volts AC
4. Check ACROSS pins 2 and 7 of V2 where the two GREEN wires attach to the tube socket and look for 6.0 to 6.6 volts AC
5. Check ACROSS pins 2 and 7 of V7 where the two BROWN wires attach to the tube socket and look for 6.0 to 6.6 volts AC
6. Check from where the RED/BLACK wire connects to the selenium rectifier (or diode if the selenium rectifier has been replaced by a diode) to chassis ground and look for 48 to 55 volts AC
If everything checks out your power transformer is OK.