In the "ol' days" before the additive package in oils was so good. Engines gunked up really bad, particularly with some of the high parafin base oils like Amalie and Havoline.
At that time, using ATF in the oil was not too bad of an approach. I even started a few engines with half kerosene and half cheap oil and ran them a few seconds with no load to flush things out.
Today, when I take any of my engines apart, they are squeaky clean because I use Universal Grade oil that is specially formulated for diesels. It has a much higher content of detergent and soot dispersal additives. I use this in my gas engines to keep them clean and have gotten great longevity from this approach. The side benefit being the squeeky clean innards.
I believe that if you see visible signs of gunk with the valve cover off, it might be worth pouring in a can of "motor flush" from the auto supply, bring the oil to operating temperature (typically about 17 miles of driving)and then drain it, preferably overnight. Then pour in some Chevron Delo 15W40 and a fresh filter. I use this oil in everything from my diesel MB's, Ford diesel tractor, MB gas engines, Corvette, wifes Toyota 4 Runner, lawnmower, etc. The only vehicle on my place that does not get this in the crankcase is my new C, which has never tasted anything except Mobil One.
I'm not proposing that you use this oil forever, but I think the motor flush, a hot drain and an oil change with Delo will have the best bet of cleaning it up with the least risk. After that you can go back to using your favorite brew in the crankcase.
BTW; When I bought my '88 Vette with 70,000 miles some years ago, it had lifters ticking. The engine has hydraulic roller lifters and, of course, the Chevy stamped steel rocker arms and balls. I have heard several of these engines make valve train noise. I followed the flush procedure I outlined above and have never heard a lifter rattle since that time.
I wish I were there to hear the darlin' run and go for a ride.
Best of luck,