Welcome and good luck
A couple of things...
First, please DO provide an image of the schematic you've modified so that any future reference to the design isn't confusing. Verbal descriptions of circuits will put down a thread faster than anything else.
Changing the filament wiring such as you propose would typically require a change from 12.6V to 6.3V. Does your transformer provide both voltages? Maybe you could indicate the tube model you propose to use for us. But more important, why use an odd tube model? Even if you have them on hand? Consider the trouble it causes. First, you'll be adding switches, complication and effort to the design. Second, modern 12ax7's have been and continue to be the most redily available preamp tube for instrument amplifiers and are available cheap and LOW IN MICROPHONICS! Something that often becomes a problem with unusual tube models used in instrument amps. Further, being as your somewhat new to this, most of the circuits you reference will be relative to the 12ax7 design. In the end, is the value of the preamp tubes you want to use worth the extra trouble?
re: Building on a Twin Reverb chassis and using the extra holes... Maybe, maybe not. Good design should dictate the control layout. Not where the chassis holes happen to be. Certain controls should, for both practical and electronic reasons, be located on the back of the amp rather than in a front panel hole. Perhaps this is unimportant to you now, but it may not always be. There are good reasons for locating bias adjustment and hum balance pots on the back rather than the front. That's why you never see them on the front.