Surely they can be inside and be fine - usually, it takes some clever steel plate as sheeting.
And nice steel chassis shell also helps.
Basically, comes down to proper engineering - something deutsch manufacturing has a habit of doing well.
And over doing even well-er
But no doubt that EM transfer happens from a power transformer at mains freq in particular, and harmonics, usually thru other transferance means. Then of course there is psu ripple and all that. I forget the V72 but I guess it has a good low-ripple psu filter.
Best defence is distance. Especially from low level circuits, such as nearby a 1:20 mic traffo! And from many-turns-inductors especially where they are not shielded.
I would try to find a placement for that transformer very well away from the input and then as far from the output transformer as well. Certainly the input traffo should be shielded. Preferably the output as well.
And yes, sometimes hanging out back is the best solution. It does also depend on what other equipment you plan to run above and/or below your preamp. Having another big tube box with it's psu traffo hanging out back but unfortunately near to this one's sensitive areas can be just as much problem.
I try to keep my psu on one side consistently within a rack for that reason. You can't really tell the real-world noise performance until the unit is in it's 'natural habitat'. You sometimes get weird interactions.
Sometimes I leave the final transformer mounting to the end in order to try some different locations. You could be amazed by the differences in noise performance you get by moving transformers around a bit.
On the other hand, I have a 'poorman 670' build with a big E-i core psu inside the enclosure full of unshielded signal traffos.
I had to place everything very carefully, testing as I go, but the unit ended up being one of my quietest, with with measured mains harmonics well below -95 dBu. (which is something like -105dBFS or so)
Anyway - carry on.
First tube box is always kind of special. I still use mine all the time - an La-2a with a fender preamp added.
Enjoy and stay hv safe