The L1049 variants did progressively increase range, including the tip tanks, but it was very dependent on the weather (and even more on the forecasters). Towards Europe, with the prevailing winds, was obviously cracked before the opposite direction. The L1649 Starliner and the DC7C were however the first to be able to do nonstops both ways with reasonable reliability (and even they didn't achieve it sometimes).
The Pan Am DC7Bs (they only bought 7) were assigned to their Miami base and operated down to South Amerca. Transatlantic services were handled by their 27-strong DC7Cs when this type came along, although some Stratocruisers lasted until the jet days because they were well liked by passengers, notwithstanding that they usually needed to stop. The DC7C had a 10' wingspan extension compared to the DC7B, added inboard, which not only gave more lift and moved the props 5 feet further out for noise reduction but allowed a significant integral tankage increase. The Starliner had a completely redesigned wing compared to the previous Connies.