This is reminding me of the tribulations I had fitting my headers that were 'new' chrome "Made in England" and FU-Jack sticker on them....bought cheapish (AU$100) for a reason...as I discovered
The top ends were both tapered in same as Trev,so had to make an expander tool to stretch those, which worked well (unlike Trev, I didn't feel I could entrust them with an exhaust shop as I don't know anyone in that game)
Then I had to cut 30mm off the same top ends as they stuck out that much too far, but then found the bend started too close in so the finned ring doesn't sit right....and the pipes still aren't seated properly- but I don't care at the moment as after six years, they don't appear to leak and runs fine, so I just use it...but that's me
What I found to be the problem after doing 'templates' of the headers on a sheet of ply (before final installation), and comparing that to a similar template of the old pipes (bought secondhand in the '70's so probably ridgy-didge), is- the original headers have a top bend of *I think* 3" radius and the new copies have a radius of 3,1/2", which is enough to completely F***ck everything, and combined with a different bottom radius (~9" from ~9.5" off the top of me 'ead- which I don't think is right anyway but close) made the job a headache like Steves....
Lucky both the mounting brackets apart from totally bodge, were not close to being in the right places, so I cut those off and welded some stainless flat bar on with a separate flat angle making 'two piece brackets'. Doing that at least made it easier to sort the other crap out....I'll fix them properly one day
Of course this is of no help to Steve, except I'm feeling your pain...