He's definitely used the '74 date in interviews, but exactly what was in these early drafts, it's difficult to say. He certainly had lots of problems with the original version (the one he mothballed) and he's joked that you couldn't understand it "without thinking in six dimensions" - but whether this is a reference to its complexity or the fact that it was a mess, I don't know. The Wasp Factory was definitely written in the very early 80s, prior to its publication in 84, and Consider Phlebas was written directly thereafter as I understand.
There's an interview with him
here where he mentions the writing of Use of Weapons, along with some general stuff on the Culture and his writing.
I'd certainly prefer to think that it was 84 rather than 74: I'd feel a wee bit less inferior...