You want the table to be flat and parallel, both top chuck mount and bottom ways. Make sure the table has not been sprung from the over tightening. First, assure yourself that the table surfaces are flat and parallel top and bottom, with equally flat top carriage ways. You can use a very thin coating of high spot blue to see the fit of carriage to table. After you make sure that is correct, or correct it as needed, you can grind the chuck mounting surface to get it flat and parallel with the ways below. You only need to grind until you have bearing at all the corners of the chuck and maybe 50% of the table area underneath. Don't grind the hell out of it, you only need to create a flat chuck mounting, not an overall mirror surface. Grind the bottom of the chuck as well to make it flat. Then put some anti rust oil or other coating on the table and t-slots and the bottom of the chuck. Make sure there is no grit between table and chuck. Do not over tighten the chuck, there is little tendency for it to fly off, and as you have found out it can warp the table, temporarily or permanently. Be gentle with the wrench. Finally, you can grind the top of the mag chuck.
To really get it right requires starting with the four upright member slides (Y axis ways), getting them square and parallel, then the wheel head housing and rear bracket plate, then the bed casting ways, the carriage bottom then top, and finally the table, bottom then top. Starting with the table can upset the geometry of the machine to where it is far more work intensive to get it all correct later. But of course this is renovating the entire machine...
See the book "Machine Tool Reconditioning" in the files section of this site.
http://www.hobby-machinist.com/threads/connelly-on-machine-tool-reconditioning.41802/
Warning, I am a newbie at surface grinders and have little actual experience at doing any of this. I just happen to also need to regrind the chuck mounting surface on my surface grinder soon and have done some studying.
Surface grinders are (or should be) very high accuracy machines, more so than ordinary lathes and mills. Try to do your best possible work on it, and try not to cause additional harm in the process.
Oh, and finally, welcome to Hobby Machinist!