So big brains behind HF Series 3 made the boxes slightly (1/8") larger than Series 2 at least for the 26" boxes (now "27"). This means you can't drop a series 3 on top of a series 2 cart... The other way around works for the 2 people out there for whom that'd ever apply.
At any rate... Craftsman 2000 series 26" top boxes fit Series 2 carts with about an 1/8" to spare. They are 26 1/16" wide. Series 1000 should as well.
Thats frustrating. I have a 72" series 2, I'm hoping the new box fits on that. We'll see I guess lol