Nilgai are about like a cow elk in size, maybe a bit bigger as some we hunted were estimated by the guides at 750-800 pounds on the hoof. They have a reputation for being very tough and difficult to keep down. As they mature the bull's coats change from elk-like colors to a unique grey-blue, no making a mistake there.
I would hunt with the .300 WMS with some tough bullets, if the factory 180 Failsafes shoot well I would use them. The bulls can be very wily and they are not prone to standing around at 100 yards and looking at you. They go and go hard, over the horizon or stop out long to have a lookback.
Although they have a pretty good reputation for tough and even being aggresive when wounded, they will die if you take out the nervous, circulatory or respiratory system(s). That pretty well means chest shots, as in any critter we hunt. Nervous system hits, as in spine or brain, are the surest but not the easiest to do.
Meat is superb, trophy is no hell - they are homely and only grow small horns about like a mountain goat. Really sporting critter to hunt, we got a couple with muzzleloaders and had to work pretty hard at it.
Hope your hunt goes well.
Had some big bulls standing out at 700+ and I was really wishing for my .300 Weatherby with 180 A-Frames.