Having looked over your stated options, and the fact that you are leaning towards one of those three options, I would choose the first option, the 273BX. My reasoning is that this option gives you a little extra ‘current’ leaway in regards to the total amperage draw from the two 2X3s. This option wil allow P/Tnsformer to run cooler should a short in some other part of the radio circuit cause overloading of the Power Transformer, causing it to overheat. I never checked all theoptions offered by Hammond, However, do Hammond not have a fourth option that would give you 5 Volts at 4 Amps, along with 6.3 Volts at 5 Amps. I know cost can enter in to the decision to purchase. Also whatever you purchase, make certain that the footprint of the new Transformer will match/fit the original mounting hole/s on the chassis itself.