[Marcello]

"stripped-down tank guns placed in concrete or stone structures"

Generally speaking when tank guns from older tanks are used the turret is retained.
The last swiss fortifications of this type consisted of surplus Centurion turrets placed inside a casemate to give them overhead cover. As I said bunkers armed with large caliber direct fire guns (like the various SPMBT gun bunkers), either fixed or field, seem to be rare in reality.
There are some dedicated fortress guns for indirect fire (like the swiss Bison)and shelters for protection against CB fire (like in North Korea).