I would think the .54 calaiber 300 grain Lee R.E.A.L. bullet - which is pretty short - would give you the best chance for a accurate conical out of a 1 in 60" barrel.

Edit: Just measured one. They're 0.615 long. http://www.leeprecision.com/cgi/catalog/browse.cgi?1198113974.2260=/html/catalog/blackpow.html