How much you narrow it will depend to some extent on what offset wheels you are using.....

What angle are the bars at when the IRS is in the Vette, parallel with the ground, or front up a bit????

Last IRS I played with was a Ford 8.8", about the same headaches though.... Ended up having to do a lot of mods on the hubs and built both half shafts.....

Lots of extra work and expense, especially for a fendered car.... If I do it again it would be on a fenderless car, and just for the aesthetics.....