Not always. My Darne R10 was a pistol grip gun, and, on that grade, the straight grip version uses the identicle lower tang.

Ditto my Tobin. Both guns were converted from pistol or semi-pistol grip to straight grip.

Neither of these were real high dollar guns. But, one could still do a bit of welding and lengthen the original lower, and inlet it into the new stock, or the rasped off area of the old pistol grip. If you can imagine it, you can probably do it.
Best,
Ted