MickyT, you must be referring to a one piece seal rear main seal rather than a two-piece seal, as in an 86 or newer block.
Well, then its pretty simple, remove the engine or remove the transmission.
Some of the same suggestions mentioned will apply.
1. Lube the lip of the seal.
2. Use some sealer about the diameter of the seal body.
3. When removing the old seal, be certain not to mar/scratch the seal interface area, ie., the crankshaft and the block. If you do, use emery paper to smooth out the scratches.
4. Install the new seal straight, using a section of PVC/pipe the same diameter as the seal body helps the install.
5. Continue to install the seal a bit below the surface to allow the lip to seal against an unused portion of the crankshaft.
Generally, the seals are installed flush, but, you suggested your's may be leaking. Hence, a groove is worn into the shaft. By installing the seal below the surface helps in re-sealing.