Early Slant 6 Rebuild

If the compression measures fine, the engine doesn't need a rebuild and you would risk f'ing up your engine. All oil seals and gaskets can be replaced from outside with the engine in the car, except for the head gasket (usually an overheating issue from coolant loss). It is a pain removing the oil pan, but after that the rear seal isn't hard to change. It isn't like new cars which require removing the transmission and flexplate (since they slide over the crankshaft to seal better). It is the same PN as used in the Hemi engine. I haven't seen the rope seal that Rusty mentions. Perhaps that was original, but my 1964 had the 2-part rubber seal.

While in there, insure that the tapped holes in the aluminum seal holder for the oil pan bolts weren't stripped by some gomer (common). Now is the time to fix that with a Heli-coil insert. Indeed, that could cause the oil pan gasket to leak right where the rear crank seal would leak. Insure those bolts are tight and don't torque beyond the spec!. If they are stripped, you might be able to screw a stud in the holes to reach unmolested threads, then a nut, or use a slightly larger metric bolt for now until some day you have the oil pan off to fix it right. I used a better silicone oil pan gasket (Real Gaskets of TN) since getting that pan off is a hassle.