When I bought the 1928 built TT Replica UE 7373 in 1967. It had been stored in the attic of A E Renolds Liverpool depot since being left for repair by a merchant seamen in 1941. In the 39 years since it was built, it had been available for use for 13 years. The 2″ LH pipe was original.
From my memory, Ken Lack used this as a patten to get some made.
When they were made and offerred to bikes, the angle of the rearward section was not horizontal. The flange plate is so narrow in relation to the overall length, that any angular error is much amplified. I remachined up the flanges to give a bolting face at the angle that gave the correct horizontal attitude for the pipe.
As the welding will distort the flange plate, I suggest to make the initial width of the flange plate thicker and to machine it up as last op to achieve both flatness and alignment.