I don't know if this is too late for you:
You don't have to go on Rt. 9 to get from Amherst to Northampton. There is a rail trail paralleling Rt. 9 between the 2 towns! The 1st time I saw it I thought it was full of broken glass, but I learned that was limestone! There is even a rt 9 underpass! It ends as soon as you cross the Conn. River. Then there is a multi-use path thru the north side of Northampton that ends at a park a few miles later. By that time, when you get back on Rt. 9 you should be much better off. I guess there will be some trucks going from Northampton to Pittsfield on Rt. 9 but most of them will be on US 20, US 5, and the Pike.
Good luck with your trip. Keep the wheel side down!