Agree the fan is way too small but that should not affect temps on a steady roll. The way electric fans are rated, you need 5,000 CFM minimum... or better adapt an OEM fan like the Chevy HHR, Lincoln Mark IV or other. Or just put a mechanical fan and thermal clutch with good shroud.

How wide is the radiator (22" or 26")? A big block prefers a 26" radiator but sometimes the job can get done with 22". It could just need more radiator.