You need to reinforce the back piece in the shell that supports the plunger tube. This is because even with the metal backplate, pressure is still being put on that back piece. Same goes for the front one in front of the plunger tube. You can epoxy metal or polycarbonate sheets to cover these spots. A 26kg spring is super strong, and will vibrate the stock shell. Buy an after market one (xzues), or put FBR on the inside of the shell. I'd highly recommend the xzues 2 shell, as it's pump action, and designed to take lots of pressure and stress from high spring loads.
Good luck modding.
I'll try to make the stock longshot shell work, but if not I'll buy a xzeus. It's minimized, and it's going to be pull back, so I would probably just sell the pump grip.