I had planned to convert the rear of the Ghia to IRS but given how tight it all is there at the moment (from a track width point of view), I'm going to stick with swing axle. I wanted to switch to IRS as it superior to swing from a camber handling point of view but bear in mind that is does widen the track - how much depends on the the length of axle you start with (i.e. short or long axle swing) and the type of a-arms used i.e. standard beetle or nice aluminium Porsche ones.
I can just about get a little finger in between tyre and arch with the current short axle swing setup I have so that's what I'll be sticking with...which also means that I'll have a built IRS box for sale in the next few days...or swap for a built swing axle...