Longest shot I'd want to have to take would be around 300 yards with any rifle on elk. Longest I've taken is 250 with a .270 Win. As far as your question about the .30-40 it has taken many elk in the past and will work just fine today. Find some loads with 180 grain bullets, I wouldn't be afraid to use one. http://www.cheaperthandirt.com/66194-7326-118.html
This should give you a clue as to how much power and range you can expect. The .30-40 should recoil less than the .30-06 that was giving you fits during deer season. Have fun elk hunting.