Nick, the Maple spalted Wreck Paul Ruby built for me featured on his site used a similar Hammond for the power transformer and it sounds great. I believe so long as the voltages are in the correct range, the PT is less critical than the OT.
I also believe over speccing the PT current handling ability just helps ensure the power supply is rock solid. The Wreck power supply does not sag in any way, shape, or form.
Actually the power supply in a Wreck (at least in my Express and Liverpool that I also used to own) sags quite a bit under load. The power supply in these things is almost as bendable as a tube rectifier setup. IMHO this adds to the clean to scream effect, basically giving the amp more headroom when played clean (higher B+) but more compression/limiting when pushed hard and the B+ drops. This isn't unique in tube amps, but is actually probably used to even more effect in an Express than most other amps. In my original Trainwrecks if you really crank the thing all the way up and hit a power chord and quickly choke the chord you can actually hear the hiss level is then quite less (than at idle) but it quickly ramps back up to normal within about 1/2-1 second. You can actually hear the sagged out power supply recovering.
I could be totally off base here, but I've measured the B+ of these things many times. Idle voltages and cranked voltages, B+ and bias. Things move around alot in these amps creating the "feel" IHMO...
You can get the same PT as the Moose's one from Heyboer with either a 240V primary or a pair of 120V primaries (which is what I did), so you can use them in series for 240V or parallel for 120V.
Part number is HTS-5199-120/240.
I'd suggest going that route so you're starting with a known quantity for your build.
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