SOME of the B+ deficiency could be gained back by substituting silicon diodes, with their lower series resistance, for the 5AR4 tube rectifier. Another slight gain could be had by the power transformer not having to provide the filament current for the 5AR4.
MAYBE by doing this you could get away with it, given the apparently better regulation of the B+ voltage at high current, where the Hammond MIGHT sag less than the Dynaco. If your home's line voltage is consistently on the high side, that might help, too.
But does the Hammond have a tap for the bias voltage? There is a circuit in Dynaco's transformer catalog which shows a way to derive the bias voltage from one of the main transformer taps, using capacitive coupling of the AC prior to rectification. I tried it when I attempted to replace the burned-out power transformer on a Mark II and it worked, but it could only provide barely enough current to properly bias the Mark II's EL34s.
If you already have the Hammond, there's nothing stopping you from trying it out. But the way the math works out: if you can gain 10% on the Hammond's B+ voltage by applying all the factors described, and if the "modern" Mark III replacement transformer gives you a 60 watt amplifier, you will still have only a 50 watt amplifier.