Has anyone tried Blue Devil Rear Main Seal Stop Leak?

Try it. If it doesn't work, you can change the seal from below fairly easily, once you get the oil pan off which is the hard part. You will need a Pitman arm puller to unbolt the steering link from the gearbox and move it down. You will likely need to unbolt both engine mounts to jack the engine up 2" above the K-frame, then prop it there w/ wood blocks under the mounts. That gives room to get at the front oil pan bolts and work the pan out. You might need to rotate the crank so #1 piston is up so the pan doesn't hit the crank arm.

The seal is on a separate aluminum holder behind the aft main bearing cap. Check that the tapped holes for the oil pan weren't stripped by a prior gomer mechanic (common) since now is the time to fix that w/ a Helicoil. In my 1964 Slant, the seal was a 2-piece rubber, but likely had been swapped. Same PN for 426 Hemi engines. If the rubber is old and cracks easy, that would explain why the Blue Devil didn't work.

Change the oil pan gasket. I used a permanent silicone gasket from Real Gaskets of TN. While in-there, I suggest removing a rod cap to check the bearing. If any wear or scoring, I would change all. Do one at a time so you don't mix them up and stamp for orientation if no factory marks (forget). Also remove a main bearing cap to check for wear. In the several Mopar engines I've done, the rod bearings needed changing, but main bearings were perfect. You must use a torque wrench to retighten the bearing caps.

Of course, before all this, check compression. If <110 psig in any cylinder, I would consider a rebuild (over-bore & new pistons or at least a factory hone and ring job). To do best, the block needs to come out. But, before that, have a mechanic you trust verify since easy to get a bad reading. Also, do a leak-down test to find if rings or valves. Plus, a sanity check by turning the engine over by hand and feeling for strong resistance from each cylinder (3 per rotation), taking >2 sec for the pressure to leak down. If so, the engine is fine and your compression test is faulty.