Managed to answer my own question - It would appear in 1977 they ran for a year with a "hybrid" leg.....
- Pre then it was all Non thru' prop, the 20 was the same powerhead lump as the 25, and they all used a spacer piece to get a long shaft.
- in '77 it would appear the 25 then went to being the same powerhead lump as the 35. New legs all round, but for whatever reason, the 35 got a new Thru' prop exhaust gearbox, and a different leg casting for long & short shaft, but the 25 kept the old gearbox / spacer arrangement, and had one leg for both long & short.
- Thus there is about a year of prduction (before the 25 went thru' prop) where you could feasibly mate the old gearbox with the 35Hp powerhead..... which would rip the @rse off my sib! (and why you would want to deafen yourself by doing that is anyone's guess!)
Net result - I can't put a thru' prop box on my old but reliable powerhead.
Bring on the ear plugs!