Ok, you're going to run into a few issues...
1. At 8 weeks old, you'll obviously be running the capsule facing backwards. Avoid dramas by buying/hiring a dedicated capsule, NOT a reversible baby seat. They take up a heap less room. They WILL fit, but your other half won't be happy, as it will have to go behind the passenger seat, which will need to be slid as far forward as it can go. Once the baby is old enough to face forward, there's more room for everyone.
2. The seats are quite contoured, so make sure that the seat fits in properly and sits square on the seat (cheaper to get a new seat than a different car ) Safety is one obvious problem, but the base of the baby seat can leave quite bad marks in the GT-R trim too - they come out eventually, but its just annoying.
3. I never ran a capsule facing backwards, but I ran baby seats in an R32 and R33 and found the R33 much easier to live with, in terms of space and ease of entry. R32 owners feel free to correct me if you disagree...
Anyway, the prices of early R33 GT-Rs are quite low these days, so it might be worth considering unless you have your heart set on an R32. Either way, be prepared for doing your back in putting the kids in the rear.