So I think I've figured out how to get 64bit working (I'm on Windows)
Here is what I did. In Steam I went to TModLoader and opened properties to brows local files. This is your Steam TModLoader folder.
Just to be safe, I copied all the files in this folder to a new location.
Download 64bit TModLoader on the front page of this thread as usual, and then drag the files into the Steam TModLoader folder. Replace files when asked.
You should now see a TModLoader.exe and Terraria.exe, either move, rename, or delete TModLoader.exe (I renamed it), and then rename Terarria.exe to TModLoader.exe
If you try running TModLoader now, there will be an error saying there's a missing image. I figured this is because TModLoader is used to running in the Terraria folder.
I went to Steam, Terraria, properties, browse local files to open the Steam Terraria folder, copied everything in the Contents folder, and then pasted it in TModLoader/Contents folder. When it asked to replace files, I chose decline this time.
Then I opened TModLoader, I got the splashscreen, the info says running 64bit, and mods started to load up. Chose single player, and loaded up my modded player and world without problems.
View attachment 271675
I hope this helps others struggling with this.