Is a standard Remington 700 short action DROP IN compatible with a 40X stock? Or is the 40X action different? I have never owned a 40X action.

centerfire or rimfire? they are originally based on the Rem mod.722 the basic action would be the same but there were different triggers and bolt shapes and most of the rimfire 40 x's were single shots so there would be a problem with a magazine

The 40X & 700 S/A are the same.

X Mark Pro trigger/bolt stop inlet is slightly different for the post 2006 700's.

I have dropped my 40 X into a number of SA M-700 stocks over the years and it works just fine.