I just did a google search for livewell standpipes. worstmarine has some with a rubber end that pushes in.
however this website has a few things shown as well
http://www.customlivewells.com/Pages/plumbingandoverflow.aspx
Thanks Scott for the info. But my drain doesn't use the push in stand pipe but the new threaded design that they are now offering. Seems folks were having problems with the push in type with the "O" rings were getting loose and falling out, draining their live wells without them noticing it and the fish were dying because of that.
However, I did a little work today to see if I could make something work. Well I did actually. But not exactly like it was designed. I turned a standard PVC pipe threaded section down a little on the lathe with a file. And after just a little turning, it fit and even threaded in. But not to the same threads of the drain threads. But it does thread in and out pretty nicely. The 3/4" pipe threads are actually a little finer then the drain threads. The drain has some really course threads and less per inch too. Here are a few pictures of that effort.

As you can see the 3/4" pipe thread on the left will not fit into the drain section. Too large in diameter.

This is as far as you can get with a standard 3/4" pipe thread adapter. It isn't going in there.

And here is a standard 1/2" pipe thread adapter. Notice how it will not even come close to touching the threads in the drain.

And here is how the turned down 3/4" thread section fits. If I turned the shoulder down just a little, it would fit flush. But I 'm not sure I want to go this way yet. There really are not the same thread compatibility. So I am still searching for the proper diameter and threads per inch pitch...
I also thought of another way to solve this problem and it is a personal way that not everybody could do easily. I can cut (or drill ) out the drain to accept a standard pipe threaded fitting and epoxy it in place. Then any 3/4" pipe thread stand pipe would work perfectly every time. And if you wanted to change the water height in your live well, just change the pipe length... Sound like a much better product then this silly uniquely threaded stuff. But then again they couldn't make money doing that. Maybe I need to market such a design and take the market... :facepalm: