6.3 volt AC will not give you 6.3 volt rectified and regulated DC. Somebody here already made a dual G9 but with AC heater and simple resistor/capacitor filtering of the HT voltage. So at least you know it will work. If you really want to use this trafo, then I would suggest AC heaters, but do the wiring off-board; don't use the PCB for this part but rather use separate wires that connect to the heaters and run as far away as possible from the PCB. The HT voltage will probably work fine, but increasing the value of the resistor after the rectifier is probably a good idea. Hopefully Jakob will stop by :wink:
On the other hand, why not build it using the transformers suggested? They are cheap, and that way it is easier for anyone who has built one here to help you troubleshoot. I think the chap who built a dual channel G9 using AC heaters and RC plate supplies did his point to point and was already good at that method of construction. AC heaters are tricky and for a preamp require very careful layout. Good luck with whichever direction you choose!
Bjorn