Sometimes if I really like a building's exterior but the interior layout doesn't suit what I have in mind, I just demolish all the insides myself and start over. Sometimes just stripping out the furniture is enough to rearrange, sometimes I also remove some or all of the interior walls.
Using the download filters to select "medium" lots for buildings on MTS, I did a quick check on the first few pages of lots that came up and most were 30x30 or 30x40, so I'd guess most of the lots that show up on that filter would be suitable for your 40x40 lots with some minor editing. (One was 35x45, so I'd guess there's some 40x40 ones in there, I just didn't stumble on any)
Hopefully that's of some help?
no power in the verse can stop me
ge smak daun, gyon op nodotaim
jus nou drein jus daun