A string bar may be the best option and also it will fill the gap cosmetically. When I get the string anchor block and the neck plate fitted I will mock up, string up and check the angles. Measuring against a standard telecaster headstock, the E strings are level with a standard A and the D and G level with a standard B, with the A and B between the standard D and G.
So I may get away without trees or bar.
Thanks again for the advice and your interest
Edit. I understand the 'scale length', but cannot quite grasp the 'musical length' which includes nut to peg and bridge to anchor (I believe) Do these extra lengths of string (which can be variable dependant on the type and size of the instrument) have any bearing on the sound, tone, playability or accuracy of tuning?