The P class divers are listed as 3' - 91/8" by Wiki - so at 1:22.5 should be 50mm I thinkI think you may find the wheels to be a little on the small side, you may have found the class 38 a bettor donor in this regard. But much more expensive of course! Look forward to seeing what you come up with.
Mind you, at 1:29, you'd need 39mm driversWould a Bachmann Thomas be a better starting point.....? Similar six-coupled chassis with inside cylinders as what you're aiming to build, plus larger wheels (I think) than the BR80...?
Jon.