My understanding is that all aircraft's datum lines are drawn from the centre of the propeller to the bottom of the stabilizer. Is this not correct ?
Incorrect. When it happens, which is rare, it's a coincidence of the design. An extreme example to disprove your statement is a seaplane with a pod-mounted engine, like a Seamaster. It has a big angle from the prop to the stab.
Pkoury is also correct, tony0707. The OP describes a 0/0 relative incidence, both wing and stab are 2.5 positive to the ref or datum line (which is highly suspect to start with).
It's very unlikely the plane has been altered significantly from the plans. OP has described it as 'very well-built'.
Put some fuel in it and go fly ! [8D]