I think part of the problem is the center stand has the least amount of material around the heli-coil, and it does not stand up the the high spring pressures. Stud or longer bolt won't help the problem because the there is nothing to grip onto below the heli-coil. It seems the other stands are OK, because the heli-coil is much deeper into the head (below the oil hole) on the middle stands, and the outer stands, it is located in a thicker cast section also.
I would recommend removing the center heli-coil, and installing a new one deeper in the head below the machined sides of the center stand. When I installed the Time-Sert I drilled 1-1/2" below the machined area and did not run into anything (like a water jacket.)

Here is a pic of the Time-Sert kit. The kit comes with drill, countersink, tap, installation tool, and 10 of the 1/2" long steel inserts. I think the kit was a bit pricy, around $60? The two 3/4" long stainless inserts at the lower right corner is what I used. They are about $1 each.