The original 51 had the reservoir location as the jack and lug wrench tool storage box location, you can still see it in the outer frame of the area the reservoir now sits in.
I presume the 57 tr3 from which the engine came had a lot more vacuum reliance than the 51 MG originally did.
Even the SU carbs appear to have a vacuum line running to them under the bodies, or at least the one carb seems to, for acceleration boost I believe.
Here’s an image of the inside cover of the Haines book capturing some of the mods the previous owner did.
Attachment 287081
Good call on the alternator conversion Tom, it’s in the notes, hehe.