You are correct. The grooves were only used in the early years, 1965-67 I think. The bearings also had a half hole per shell to let the oil through. To get the benefit of the rods you have to use the correct bearings. I don't know if anyone still makes them.My stock '65 396 engine has an oil groove in #8 rod cap but none of the others. Is this common? I read that the groove is meant to throw oil onto the cam and cyl walls.
Chances are most of your rods have been replaced. I don't know how much the groove helped but it falls in the can't hurt category.