Will a ba/bf coolant overflow resevoir fit on a pre au ?

I don't think it would be a straight bolt on swap. But that is not to say you couldn't modify one to fit.
From what I can remember of the BA header tank the main outlet is in a different place and the mounting points are different. Not sure what radiator hose you'd need to use either.
Sounds like an interesting mod.

it can fit, the inner guard shape is the same but you will need to drill holes and change the hoses..

sweet thanks guys