Two problems, cracks mean loss of energy. If you do not want to remove the floor and install a subfloor, then I would first make sure all the areas are solid re-enforce with additional support in weak areas. Then I would install batt insulation and I used plastic netting to support it stapling against the joists. You can also use wood strips. Make sure you overlap the barrier and staple it to prevent air or drafts, very key. Now you have a more efficent house. I really wanted to remove the pine floor and put subflooring down but you are talking taking all the base molding up etc. and is usually done if the floor needs repair anyway. hope that helps. The insulation is well worth it. dont fill the crack unless you are resanding. and be sure to use a special putty for floors. I would not use for seam cracks but for indentions holes etc. the seam cracks will work the putty back up and you will have a mess. Good luck.