I went to Woolf Mufflers in Dominion Rd Auckland to replace the standard cast iron manifold on my 5MGE with a set of extractors that had to come in from Australia.
They didn't fit properly and had to be modified. Australia only seemed to get the 5ME.....
It might take a while but you should consider making your own. I did this for the second set and the local welder gave me some useful tips on assembly. I think it took a little over a month as I don't have a welder and would pop up and see him on Saturdays for welding.
I now have an equal length (primaries) extractors and with the HPC on them, I'm more than happy.
David.