What kind of vehicles are your SBC's in, are they also swapped into older-style hot rod vehicles? It sounds like you have an airflow problem, got any pics of your radiator and fan setup? I've always run factory thermal clutch-drive fan and shroud and never had temps creep on the freeway going 80 MPH in my '70 Duster. Might be a case where the shroud for your electric fan obstructs free air flow when you're driving at speed. I'm assuming a mechanical fan isn't really an option though with your custom setup?
