Nice work Liam, It's really coming together.
I made a separate set of headers for the under car exhaust. I figured i didn't want to make too many compromises with the side pipe headers.
My undercar headers were based on Corvette exhaust manifolds (cut down to fit). They were cheap to buy and tucked in close to the block. Several folks have used them for rego now in fact I'm not really sure who has them at the moment.
If you are careful with the routing of your pipes you can make the headers pretty easy to unbolt and remove. I can unbolt mine and take them out through the engine bay pretty easily. I also went with 1 7/8" primaries as they suit my cam and intake combo.
Cheers