Need advice big gaps wood floor and dirt floor basement

[Tammy Richards]

Have a 4 unit multi built @1900.  Wood refinished floors have huge gaps/cracks, and they are layed over what appears to be what was called a subfloor in the Victorian era, which is wood which also has gaps.  From the basement, I can see straight through these gaps to the apartment.  The basement has a dirt floor.  I want to lay floors (hopefully myself) on top of the wood floors, to improve air quality and cleanliness - vinyl plank will be fine for my section 8 tenants.  Would like to lay vinyl plank or floating engineered myself  I'm fine with filling gaps from below the basement in with Great Stuff or something, but hoping to avoid laying in a plywood subfloor (I don't have the tools, equipment, or time for that). Wondering if sealing gaps with great stuff and putting an underlayment which is also a moisture barrier would do?   Flooring is pretty level, and in good condition.   I'd appreciate recommendations/advice from folks.