Wow, Bob did a beautiful job on that rig!
Yes, a pair of p-p triode-connected 813s would be an excellent tube choice to modulate a single 813 final. Use a 1:1 ratio mod transformer, though a 1.5:1 or 2:1 step down would be OK too.
I have run a pair of 813s modulated by a pair and this combo was possibly the cleanest plate modulated rig I've ever run. No driver transformers, less phase shift - just the mod transformer with plenty of NFB from modulator tube to early low level stage.
I used a solid state WA1GFZ FET driver for the 813 modulators. SEE for FET audio driver details: http://amfone.net/Amforum/index.php?topic=23632.0A pair of tubes of the same type modulating a single final has always been a choice which gives plenty of audio headroom. You want overkill so that the modulators are just loafing along which keeps them in the cleanest part of their transfer curves and far away from saturation.
The 813, 4-400A and 4-1000A (tetrodes, pentodes) are a select group of tubes that operate as well as real triodes when connected as triode-connected. Connect the grid and screen together of each tube and drive them p-p. Add a few diodes in the filament CT for bias to adjust the plate idle current. 813s are super rugged and plentiful. They are rated at 125 watts, but I believe the carbon plate versions are really more like 150+ watt tubes and a little more with some air in a chimney.
Check out this sample rig using 813s, a pair modulating a pair:
Under extreme testing conditions with 3KV on the plates, this combo put out 900 watts of carrier and modulated cleanly 140%. Unbelievable tube, the 813. 2200 volts would be a nice level for everyday use.
http://www.amwindow.org/tech/htm/813/813.htmTom, K1JJ