Welcome! I would like to start out by saying that I am in no way an expert on the 250 GTO conversion, but I may have some general advice and a couple questions. How comprehensive will the rebuild be? I would probably recommend you get a later 280Z for your donor. My reasoning is as follows: I am assuming that you are keeping the Northstar V8 fuel-injected, therefore I would sat go for the 280Z because that's less work you would need to do on the fuel system to get it compatible with the EFI. My other reason for recommending the 280 is because of availability and price. Since more 280Zs were made than any other S30, there will be more and they should be cheaper because they are not as highly sought after when compared to the 240 or 260. I say later because more people seem to go for the '75 because it is smog exempt (at least that's the situation in CA).
Anyway, hope that helps!