pinkflyd34:
I'd be concerned about the size of the plate versus the size of the concrete pier. You should have a pier that is very nearly the same size as the mounting base of the scope's mount. --Its about 3" over the edge all the way around. Concern????
My concern is that the plate extends so far beyond the pier. It appears the pier is undersized. It sounds like you'll be putting about 60 lbs of weight on it.
However, if your mount's base does not extend more than an inch or so beyond the diameter of the pier, it's not really a problem as long as you're not putting any significant weight on the outer edges of the plate. So, for example, if your plan is to drill holes in the plate and use them to hold eyepieces, it wouldn't be a problem. But if your mount's base is as large as that plate, then you'd be putting a lot more torque on the pier, so it would vibrate.
If it turns out the concrete you have now is well set, and not cracked, and that the outer diameter of the pipe is about the same size as the mount base, then you might be able to get by with it as-is, simply by welding some threaded rods to the outside of the pipe and extending them up through a (smaller) flat metal plate. Or maybe simply by welding a bead all around the top of the pipe, connecting it rigidly to the plate. I don't know if that pipe will take a weld, however.
In technical terms, the torque/moment arm on the pier decreases as the 4th power of the increase in pier diameter. Thus, doubling pier diameter decreases the torque (and therefore potential vibration) by a factor of more than 16. Conversely, if you decrease the diameter you increase the potential for vibration and instability.
If you measure the base of the LXD75, the pier might be very close to that diameter. However, remember that is a long scope (compared to an 8" SCT, for example) and the counterweight shaft has a lot of weight on it. Therefore, the torque you put on the top of the pier is increased by the "moment arm" of the scope, and that will make it more susceptible to vibration.
I don't know what the exact dimensions of the "golden ratio" for your setup will be. But I think it would be better if that pier were at least 6" and even better more like 8" in diameter.
You also mentioned that the pier is "3 ft. in the ground ... with a ton of concrete around it" ... If the concrete extends to touch the surface you'll be walking/sitting on, then there's an additional source of instability.
Given what you've told us thus far, I think if I were to fix it I'd get a 10" sonotube and put it down over the pipe and cut it level with the top of the pipe or perhaps a couple inches higher. Pull out the 3/4" threaded rod. Get six 18" long threaded rods and have the lumberyard or hardware store bend one end of each into a 1" elbow shape. Then bolt those rods into a piece of plywood, in a circular pattern midway between the outer edge of the pipe and the inner edge of the sonotube. Fill the sonotube with concrete (you'll need to brace it at the bottom to keep it from moving and leaking concrete) and then press the plywood form and bolts down into the concrete while wet. When the concrete hardens, you can remove the plywood and replace it with the metal plate. This would give you a much larger pier relative to the mount base and plate, and it will be much more stable.
Ask someone who works with concrete for a living about this. It may be that the iron pipe should be coated with something before pouring concrete around it. They might also recommend a different mix of concrete than, say, Sakrete for such a job. I don't know about that. I do know that concrete contractors vary the mix of concrete depending on how much reinforcing iron is in the forms ...