Well, there have only been 4 SBC 28ATs built. Chris Spillman claims awesome mileage with his twin 350s. You can get a hold of him here:
Mileage though is all relative to how one drives; especially on the single. Mileage at 3000 vs. 4200 will vary greatly; as everyone has a different cruise RPM.
'Boatfreak' here on OSO can give you some input on his former 28 with a 496HO. More bad news beyond the economy