Will a harmonic balancer from 302 fit a 351 w?

Yes, I believe it will fit. BUT, the timing marks won't be the same. Not a big problem, you'll just have to put new marks on the new balancer.

Yes it will fit but it might not work as there are 2 differently weighted balancers

1st balancer is a 28 oz imbalance used on 70-80 302 and 70-1997 351W 2nd balancer is a 50 oz imbalance used on 81-90 302