Hullo Ken.......
I think I understand your quandary....having said this, I don't think the
red spanner + bending will be the ideal way to attain that 6 degree tilt...the result would be a crumpled dogs breakfast
1. If you are comfortable with silver soldering, cutting out that annular elliptical wedge and the following mating together & silver soldering may offer a solution
2. You could also consider 3 support strips say 6 wide x 20 long x 1 thick brass strip......drilled with M2 clearance holes at top & bottom....bent to suit the 6 degree tilt.....& having M2 holes drilled & tapped into the copper chimney stack around the joint........these would give you all of the necessary support during the soldering process....they could also be soldered in & wouldn't look out of place
3. Another option could be one of those 135 degree Yorkshire copper tube bends .....cut & shut to achieve the 6 degree tilt
Another thought from left of centre
4. This may only be suitable for a vertical [gas] boiler ........place a 6 degree angular cupped round wedge the diameter of the boiler under the boiler base to achieve the 6 degree tilt.... [I cannot see any downside
around the boiler function apart from ensuring the water gauge was on the high side]
Good luck.......... Derek