just installed a B&M trans temp gauge on my new trans, and their instructions say anywhere from 150 to 250 is normal. the gauge turns yellow at 220 and red at 250.
the 727 specialist that built my trans says 180 to 200 is fine but may go to 220 on very hot days or with lots of city driving and a high stall converter which raises the temp because of lots of slippage at low rpms.
i installed a aftermarket trans cooler in front of radiator and hope to keep temps below 200 all the time. we will see, should drive the car again this week sometime if all goes well.