I agree that the crack probably isn't the cause of the oil leakage from the rear. It doesn't help, but that's not why the oil is leaking out. Besides checking the clearance of the rear main bearing, just as important - if not more so - is center main bearing's clearance. That area wears faster than the other two main bearings because as the fulcrum for the spinning crankshaft, it takes a greater beating than the front or rear main bearing caps. The crankshaft flexes in the center as it turns, which causes a whipping effect with the center main bearing taking the brunt of this whipping action. That wears away the Babbitt bearing material faster than in the front or rear. Because of this, the center bearing will have more slop in it, which in turn affects the wear on the rear main bearing surfaces. Generally in an engine with a lot of miles on it, the center main bearing will need more shims removed to bring it into spec's than the rear main will. The front main bearing rarely needs more than one thin shim removed unless the engine is REALLY worn out. Get the center main into spec's at 0.0015" clearance first and then adjust the rear main cap for the same clearance. Double check the front, but I'd be surprised if you are able to remove more than one thin shim there. Let us know how many shims you have removed from each bearing.
Another thing to consider is crankshaft fore and aft play, meaning how much thrust surface is left on the rear main bearing. If the thrust surface is worn, the crankshaft will move back and forth while turning, which in effect causes a pumping action that sends oil through the place where there is least resistance to stoppage = out of the rear main bearing cap. You can actually see the crankshaft moving fore and aft while the engine is running if the thrust surface is worn away. Watch the crankshaft pulley's movement. If it moves back and forth more than a fraction of an inch, the rear main bearing thrust surface is too worn to keep the crankshaft centered in its bearing saddles. Even with the main bearing caps adjusted properly, this pumping action will still force oil past the rear main bearing. It's easiest to check the thrust play with the engine out of the car and upside down (preferably on an engine stand) where you can push and pull the crankshaft back and forth with the bearing caps in place. If there is too much slop and the engine is out of the car, you can install a brass thrust insert in the block sold by vendors, but the engine has to be out to do this. You have to sand/file the surface until the crankshaft can be spun with resistance. It's a pain to do this, but it will save you rebuilding the engine just to restore the crankshaft's thrust play. I have done it and the engine is still working fine with the minimum allowable leakage. If the engine is still in the car, use a long pry bar to force the crankshaft back and forth. If you can see or feel movement fore and aft, the thrust surfaces are worn.
As to the crack in the Babbitt, of course the best fix is to have the bearings repoured and align-bored, as already suggested. That doesn't seem to be an option for you at this time, so try the next best thing as a stop-gap measure until the bearings can be repoured. Take a small triangle file and carefully carve a "V" trough into the crack along its full length. Don't go any deeper than necessary. Once a trough has been filed through the crack, use an aluminum epoxy or similar tough epoxy and fill the crack with it, smoothing it out as much as possible on top to reduce the amount of sanding required after the epoxy cures. Then carefully sand the epoxy until it conforms to the rest of the bearing surface, checking occasionally with Prussian Blue. Sandpaper on a large wooden dowel or rolling pin works well to maintain the contour of the bearing. Reassemble between sandings and test the clearance, as well as resistance to turning. Be sure to oil the bearing surface before final tightening. I have made this temporary repair to a couple well-worn engines and it works a lot better than allowing the crack to get worse and maybe start losing chunks of Babbitt. Start squirreling away pocket change to someday afford a Babbitt job so that you can make those long-distance drives that this cracked bearing cap is preventing you from doing now.
Marshall
