Maybe it's that you can't find a sweet spot for that powder. Try switching powders.
1970's barrels might be good or they might be rough. I'm not suggesting that your bore isn't in good shape, but have you considered taking it to a gunsmith, and having the gunsmith check the condition of the bore with a camera? If it's pitted from some neglect, or badly copper fouled from lack of proper cleaning, then you will never be able to get the groups to tighten up properly.
Another item to check is the screw that tightens the receiver to the stock. That screw is supposed to be torqued to a specific torque. If it's loose, or not to proper torque for that gun, the receiver will move slightly with every shot, and the groups will suck. That's an often overlooked detail. Be cautioned that the proper torque for that screw on your gun is in INCH pounds, and not foot pounds. A gunsmith can help you with that. I had to buy a special torque wrench driver to set the torque on my rifle receiver screws. It's adjustable in INCH pounds.
Also, the "FREE FLOATING" or to the opposite end of the spectrum, the "Bedding" of the barrel to the stock will also affect the groups. As the barrel heats up, if it presses against the wood in some wrong way, it will change the alignment of the bore ever so slightly. Off the rack Rem 700's are notorious for barrels pressing against wood. I changed the wood stock on my 700 out to a synthetic free float barrel style stock. My barrel now rings like a bell every time I pull the trigger.
Finally, a recently cleaned and lubed barrel won't fire accurately. Wipe any oil in the bore by running dry patches down the bore prior to shooting. And then, fire about ten shots to clear any remainder of the lube/cleaner from the barrel. They call those "fouling shots". After that's when your rifle is most accurate.
I'd go through the list of details that I have just suggested, eliminating them one by one, and then start working up loads.
Please report back.