I don't see the PSD-PTB coalition surviving for much longer(I'd say until the mid-70s); PTB was quickly gaining terrain over UDN and PSD as Brazil became majorly urban (Brazilian population became 51% urban around 1965), and would soon become the dominant partner of the coalition, which means they would try to enact policies that would clash with PSD interests(land reform, I'm looking at you). When the divorce happens, UDN would be, barring some unexpected event, the natural partner for PSD.
A PSD-UDN coalition would have a less-defined senior partner, as while UDN would still be growing(as it was urban-based) and PSD shrinking, the ratio of shifting would be smaller, as UDN's base(urban middle-class) was naturally smaller and slower-growing than PTB's(urban workers). The PSD-UDN coalition would also be less dominating over PTB than the PSD-PTB one was over UDN. I'd expect UDN to slowly absorb sectors of the PSD and PSD become a small party(or even disappear eventually) as the 1990s arrive, meaning we would get almost a two-party system as PTB and UDN dominate politics, with each one having a small constellation of allied minor parties.