Wildebeest are about the same size as impala, body shape a bit bulkier.

DoubleD - I'm not trying to stir anything up here, but Blue Wildebeest are much larger than an Impala: well in excess of 350 pounds, while a male Impala will run @ 125 and a bit more. The Black Wildebeest will weight in between 200- 300 pounds. Your choices of caliber are good. A 30:06 with a well constructed premium bullet of 180 or 200 grains will take down any plains game available.