What will happen?
 

Guys I am wanting to get a new MAF for my 2000 expy. I was thinking of going with a 90mm Lightning MAF. Please correct me if I am wrong, but arn't they stock calibrated for 42lb ijectors? If that is the case, what will happen in the motor with my stock 24lb injector? Will it run lean or just push the 24s to the limit?

First question is for what reaason do you want to change?
If it's for power increase by it'self.....don't do it.
Your getting into a change in an engineered system that requires more than just these parts changes.
The PCM still will treat the change as if it was still stock parts, without a program change.
Unless you using a lot of WOT specific operation, the change won't buy much improvement, but problems.