First off, it's important to know WHY the boards 'did what they did'. Over the years, they swelled and shrank as a result of expansion and contraction (E&C). The E&C is the result of drastic fluxuations of humidity. The more humid the room, the more expansion. The LESS humid the room, the more contraction. In your case, with the floors as OLD as they are, you essentially are working with 'raw wood'. Today's flooring is much more 'stable' because of the modernization of the industry.
Now, where does that leave you? The BEST way to fix this is to remove the planks and relay them. HOWEVER, by the time you're finished, you'll probably be a plank or 2 short at the end. You can lay there what you want to fill the gap, and kinda 'cover' it/them w/furniture, or something. OR, you can use planks from another room or a closet or whatever.
You'll hear 'ideas' about filling the smaller gaps w/sawdust or even rope. Well, again, over time especially if your home doesn't have its humidity under control (in the HVAC system), you'll be back at it again in a few years. The dust and rope end up getting compressed and 'pushed out' over time. Consider finding boards at a Salvage Yard. If you need leads, talk to you local Preservation Society or Historical Society. There may be some good leads from them.
My best to ya and hope this helps.
Jay J -Moderator
PS: IF you relay the boards, you should get the home's humidity 'under control'. If the house 'leaks' air, you need to address that. I'm obviously not aware of the overall condition and details of the home, but 'lock it up' a good bit and get the humidity under control. Once the house is in this condition, the boards will move VERY little. THat's OK - It adds character ...