We have a 403(B) plan, a 401(k) plan & an integrated MP plan. We plan on eliminating the 403(B) plan, keeping the MP plan & amending the 401(k) into a safe harbor 401(k).
Could we structure the 401(k)match so that it meets the ACP safe harbor & then make the 3% non-elective contribution into the MP plan? If so, could the MP plan still be integrated?

slow down a bit. which safe harbor are you going to make...the 3% nonelective or the safe harbor match?
you can certainly make the 3% nonelective in the money purchase plan, the 401(k) plan must state that the safe harbor will be made in the money purchase plan (at least that is my understanding of the rules)
the safe harbor can not be used for integration purposes.
The mp could still be integrated, but just don't have the safe harbor nonelective attached to that portion of the contribution.