Here is the situation:

There is a small business owner who would like to adopt a plan where he can maximize his contributions. He doesn't have a problem matching, but would like to limit the matching to only the full-time staff. He has over 150 employees but the majority of those work marginally (PT to less than PT) throughout the year.

I thought a Safe Harbor 401(k) best, but (now here's the simple question) even after reading IRS Notice 2000-3 I don't know whether or not it is even possible to limit those who are able to participate in a Safe Harbor 401(k) by age or hours worked?

Any plan, including a safe harbor 401(k) can require that an employee meet age and service requirements to participate. The maximum age is 21. The maximum service is 1 Year of Service, which can be defined to require up to 1,000 hours.