Ping at cruise speed

What year engine ? Was it originally equiped with an EGR valve ? When a EGR valve is cancelled the combustion temperature rise and help pinging to happen. Enriching the fuel may help combustion temperature to lower. 

The EGR does alter the combustion mixture as an attempt to dilute the amount of fuel being consumed, and in the end reduce total emissions. Any change from the original mixture going into the chambers should need an adjusted timing and fuel. The 70's engine were inherently poorly cammed with low compression. It's like an amateur camshaft designer was trained to design cams for high compression, and then forced to create something for low compression. It takes a better cam design than they knew how to make back then, for low compression with EGR and catalytic convertors.

So just for reference, the noise I was chasing when I started this thread turned out to be a cracked exhaust manifold. I also ended up going to replace the rear main seal and finding that the crank had been forced into the thrust bearing to the point that it put a groove into the crank, which, even with a new thrust bearing put in, still allowed about 1/8 inch of endplay, so I found another crank for it, which was 30 under on the rods and mains, so I pulled the engine, replaced the bearings and rings, and honed the cylinders. I also replaced the rod nuts with ARP ones and the valves with one-piece stainless ones. I reused the cam and lifters because at the time, new flat-tappet cams even from reputable brands were often not surviving the 20-minute break-in without flattening a lobe and putting metal through the brand new bearings, and a roller cam and lifter set wasn't in the budget, so I figured I'm better off with a known working 50-year-old cam than a new one that might ruin all my work before I drive it even one mile. I put on new exhaust manifolds at that time, and got rid of the particular noise I was chasing.