WE encounter it almost EVERY TIME we replace a tub. If the old drain is in the same spot as the new one we rejoice in our good luck. There are several possibilities, but what you have now determines the best solution. We need a picture to even have a chance to help you, but the first part of ANY solution is to remove the old drain. The way it stands now it is NOT going to work and raising the tub and/or furring out the walls, is a hack job and will ALWAYS look like a hack job.