It would take more work than dropping a turret into a ring, but the turretless like tank "STUG" was the most successful tank in WW2 and could be easily made from a M113 Gavin.

 

www.geocities.com/armorhistory

It was a successful assault gun in its various incarnations, 3 or 4 different 75mm guns up thru a long L70, and a 105mm howitzer in one model  (better than anything open-topped like the Allies had),

but it wasn't a tank.