Here is where I am at on this journey Jim's downstairs bathroom project
although I have made some progress since the latest pictures posted. Are you talking about an alcove tub, drop in or freestanding? The alternative to moving your shower drain would be build your tub alcove/frame around the old shower drain, space providing. Also tubs typically use a 1 1/2" drain whereas showers typically use a 2" so you are probably going to have to do something whth that old shower drain. Another one of those "pictures would help" scenarios I think.