An extensive experimental campaign has been conducted at EUCENTRE to understand the dynamic behavior of historic stone masonry structures and evaluate the seismic performance of selected strengthening strategies, aimed at improving wall-to-floor connections and in-plane diaphragm stiffness. Shaking table tests were performed of full-scale two-storey buildings in undressed double-leaf stone masonry with timber floor and roof. A first prototype (Building 1), representing a vulnerable building without antiseismic detailing and devices, was tested showing a response characterized by in-plane distortion of the flexible diaphragms and local out-of-plane failure mechanisms. In Building 2 the wall-to-diaphragm connections were improved, providing only a moderate in-plane stiffening of the wooden diaphragms. When subjected to shake-table testing, the strengthened building showed a global type of structural response without the occurrence of out-of-plane mechanisms. In the present paper the strengthening interventions on Building 2 are described, and the results obtained during the dynamic tests are illustrated, focusing on the response of the structure, the evolution of damage mechanisms during the tests, in comparison to the seismic performance of the first unstrengthened reference prototype response. The improvement of the connections proved to be very effective, increasing significantly the seismic capacity of Building 2 with respect to Building 1.