This thread prompted me to do a little research, so I broke out my GM Illustrated Parts Catalogs, started digging and running part numbers online. AFAIK, all SBC and BBC have the same thermostat hosing and water pump inlet sizes. The water pump inlet is 1-3/4". The thermostat housing is 1-1/2". In 1980, GM switched the G body radiator inlets from 1-1/2" to 1-/14". Every 1980+ replacement upper hose I've looked at is 1-1/2 on one end and 1-1/4" on the other. The 1978 & 1979 had 1-1/2" on both ends of the upper hose, so I can see why people might have a problem with getting the right upper hoses.
Lit549, any stock replacement lower hose for a 1978-1983 Malibu 5.0 V8 should fit fine.
Melloelky, I have never seen any GM G body radiator with 1-3/4" outlet. IDK what brand and application your radiator is, but I'm assuming it was some sort of "universal" or retrofit application.
In any event, for anyone looking at radiators and hoses, you want a radiator with a 1-1/2" inlet and outlet and you want a 1978-1979 upper hose.
Tommy