OKC apologies, I'm not - nor have I ever been - a 737 pilot. I assumed that Gonnabe's reference to the 'classic' meant the fully-clockwork machine.
However, the point remains that there is little point in getting a rating on an aircraft you're not going to fly. If a -400 rating is interchangeable with an -800 rating, there's no problem. If it's not, then it's worthless if your target airline operates NG 737s.
Scroggs