if you are binning the centre piece then you have a few options.
that is if i remember correctly and the centre section goes over
the manifold piece
you can heat up the centre pipe, that should loosen the joint.
or you can carefully grind the slot further down with a thin angle grinder cutting disc, i use the 1mm ones that cut much quicker & neater. just be careful not to go through the manifold bit though
if you don't fancy grinding it then a chisel often peels it away, but is more effort
at work we use the accetylene torch to blow a slot down then they come apart easy.
or stand it up and drizzle some wd40 or similar in the joint and in the morning it will all slip apart easy
best off to use some gasket paste in the joins when you slip it back together, saves the annoying blowing. make sure it's clean & dry when re-assembling and you should be fine
did you get all the mounting clamps/brackets with that centre pipe? only i found one today that looks like it should have been with it maybe? is there a clamp missing where it joins to the back box?
i will send it to you if it's missing, if it's from that pipe