You are talking about the M113FSV. They mounted a turret off the Saladin armored car with a 76mm gun.
Correct, and known as the *beast* or *beastie* by the lads out bush. The later FSV follow-on was the M113MRV with a Scorpion turret, also with 76 mm gun, and with better sights, a rangefinder for the main gun and passive night vision equipment. But the Sheridan turret version was also developed, though I'm unaware if it was ever deployed or saw combat.
My foggy old memory tells me there was also a prototype M113 fitted with a Bradley turret as well- can't recall if it was one of the stretched versions or earlier shortys, but the old aluminum bathtub has been so modified and hotrodded over its life, most anything is possible.