The only need for the longer case was in the black powder days, when the only way to go faster was to use more powder. As you have found, you can make a 45-90 equivalent load in the 45-70 case. The long cigar cartridges do look sexy though........
Were I you, I would throat it, but there may be a difference in the throat for cast bullets as opposed to jacketed, it may be worth asking. It will also be an easy job to do yourself as minimal material needs to be removed (so, be Extra Special careful).