The XJ-S has a rear track of 57".
The MGB has a rear track of 49-1/4".
Unless you do what Blackwood did, and extend the fenders out, you will be faced with fabricating new upper and lower arms for the IRS and having new half-shafts made up.
Probably some outfit that will do that for you......or already has.....but I think the $200 for the complete IRS is going to seem like spare change.
Most likely an automatic, and if you have an auto in your MGBC (MGB Custom), okay.....otherwise, even more money to change the R&P.
Wonder with shorter shafts and arms, how much more frequently the u-joints will need replacing?