Can I replace my 383 rear main seal with motor still in the car?

Well it appears I do not have an oil pressure sending unit. One side is plugged and the other side just has a nipple on it with the pressure line running through the bulkhead to the oil pressure gauge. Doesn't seem right to me. But it is dry at the top of motor where the sending unit should be so apparently it was my worst case scenario and I will have to replace the rear main seal.