Id roll the edges of the x pipe slightly to give the 2.5" pipe a tighter fit around the outside of the 2.25" pipe and just fillet weld it around the outside. The 2.5" pipe should fit pretty tightly around the 2.25" anyway. It will add a different sound to the exhaust as the section behind the x pipe will gargle a fair bit on decelleration due to it being more free flowing.

Hey Alan96, thought I'd add to your post if that's ok?
I started out with the intention of building my own but after spending all day last saturday fitting the headers then running out of bends late sunday, getting some more on monday and running out of straight. Went to get some straight pipe on wednesday from the exhaust guy I know, who says' "why don't you just buy a system?" and then tells me the price. So I say "Now you tell me!?" I ended up buying an X-force system because the price was just too good to be true!?
So anyway I fitted it friday night and it fitted very well. Only thing is, it's hanging a little low, maybe not a problem on standard or slightly lowered cars but mine is ridiculously low so I'm going to have to mod the hangers which means mods to the flanges on the cats. So I'll buy some of the 3" body xforce cats instead of modifying the FG cats(again!?).

I would have more photies but the camera went flat

Stock headers...


Not a lot of space but more than the right hand side!...


Di-Flipos on. Let me tell you the instructions are very good except for the ones about the steering heat shield....



Stock vs Di-Flip...


Here's what I had to work with at the start...


Here's the FG cats I modded which is a lot of work, in fact too much!


I'll post some more photies when I get some after I've modded the height.
Anyway it sounds good(nice crisp sound) if a little quiet for my liking but I can always take the rear muffler off. Plus it'll burn in a bit and get a bit louder.
Dyno'd it yesterday and it made 356hp with the wheels spinning a little(tyres are a tad bald )
Torque is improved also. Driving it late last night up a hill it lit up at the top of second gear