66 Cuda Oil Gauge

You are suggesting 2 different size holes in engine blocks for oil senders. That doesn't seem right to me. If you do need a bushing, so be it. You want nearly zero resistance between the body of a sender and engine block, and neg' battery post. Most of the new senders I've purchased already have a very light coating of sealant on the threads. A brass bushing in cast iron threads should get the same. A little brake cleaner will remove oil from threads in the block. Teflon tape could interrupt the ground path.