Anyone know of a neat way to go from 16mm (Bottom connection on my header tank) to 6.5mm (R1 pipe near propshaft) ?

Check out Car Builder Solutions they have a variety of straight reducers.

You could use a 15mm to 8mm plumbing straight adaptor.

If you use silicon hose the 6.5 should stretch if you heat it up in hot water and use lots of lube...... The 16mm should be okay (again, dunk it in ya tea first!!)...

thanks!