You might have several PODs for this one. If you want to play it safe, handwave a storm and sink Ramón Freire’s expedition to Chile. This is really what gave Diego Portales the perfect cause to turn bellicose against the Confederation (and btw, his allegations of Santa Cruz’ support for Freire’s planned coup were never validated.) If you want to be risky, you can use later PODs: Santa Cruz arrests the entire 1st expedition (not likely knowing him) thus depriving the Chileans of vast amounts of manpower and munitions used in the 2nd expedition; he approves the proposed invasion of Chile in late 1838 after the defeat of the Argentines; he shatters part of the enemy army when they were evacuating Lima, also in late ’38, and re-embarking to flee north, as many of his generals begged to do; or as others stated, a victory in Yungay.
A triumph in Yungay, will eliminate the prestige, and practical strength, of the Restauradores, and depending how many opposing Peruvians are left, the war might linger on for a few months. After that…it’s over.
As for the future, you might have it vie with Brazil for SA’s #1 spot. Remember that at this point, large tracts of land that later became Brazil’s are still nominally Confederate. Knowing Santa Cruz, not an inch will be handed over freely, like several Bolivian and Peruvian governments later did IOTL. Also, Peru is about to enter the guano bonanza that financed incredibly amounts of development (and corruption) in the country for the next 30 years. Even later, came the saltpeter boom for both Bolivia and Peru. In short, the Confederation won’t be cash strapped.
And if Santa Cruz (with his superb administrative skills) manages to have the Confederation’s founding law approved, you have a Chief Executive that remains in power for 10 years, guaranteeing enough stability for the project to solidify.