Just because people have done this mod for the past 40+ years doesn't mean it was a good idea to begin with...
It is common practice to modify the backplates on early Minis equipped with twin leading shoe front brakes to accept the later, larger front cylinders.
According to Delphi, the holes in later (27H2013/4) brake backplates were intentionally relocated compared to early ones in order to prevent the use of the later wheel cylinders in the early back plates. Presumably this also means the later wheel cylinders had their rectangular locating boss correspondingly relocated to accommodate for the new location on the back plate.
As a result, there is a possibility this modification could have some safety/liability issue. Just because people have done it for 40 years, it does not mean there is no chance some problem during use could arise from this conversion.
Therefore, we do not recommend going to larger bore wheel cylinders when replacing early wheel cylinders as a set. If someone with early drums wants better brakes, they need to fit late back plates at the same time as fitting the later wheel cylinders.