I plan on re-locating the header tank to the OS of the vehicle. As a result what I want to do is remove the pipe, tap the housing and screw in a 90 deg bsp-> 10mm hose adapter.
The "new" tank I have has a 10mm fitting for the overflow, so rather than having an 8mm-10mm adapter in line I'd like to use 10mm ID pipe all the way.
So it kinda solves 2 problems at once for me - the size and direction of the fitting.
I had thought you were contemplating something like that.
Are you going to increase the size of the (pin) hole in the thermostat housing?
When I looked at fitting a header tank I discounted the offside because of space limitations
even though such a location could make the hose routing easier.