Curt asked for a description of how to do overlays on Google Earth. I started the thead below for replies.
I am doing this from memory so this is subject to brain lapse
Scan or acquire a digital map you want to overlay. Crop it to the area you want to examine. Big maps with small areas of interest are hard to manage when you attempt to align them.
I don't recall the exact sequence for the overlay process, but there is an overlay button at the top of the GE page. Click it and it probably asks for the file location (link) on your hard drive. Adding the overlay is easy, aligning it with the underlying GE map is the challenge.
First, the overlay will have controls (green) for moving, rotating, and resizing the overlay, and there is a slider in the side bar of GE to adjust transparency. Shift needs to be held down if you resize and want a fixed aspect ratio (you probably do!)
The most obvious task is to match two or more known points on the overlay to the same points on Google Earth. This is harder than it sounds, but very doable with time and a little practice. And as you know, old maps are not always spot on, so sometimes it is a compromise. They are often very good at the micro level, but miss the boat when you are comparing large areas, then looking at the detail. Anyway, “the proof is in the pudding” (old term!). And “practice makes perfect”
One thing I value is a service called Earth Point that superimposes section and township lines on Google Earth. It costs a few cents each use, but it can save a lot of time if your map has section lines.
Use this as a start and we can answer questions here that others may have and get advice from the pros.