I typically use two pieces of 0.063? 3/4? x 3/4? aluminum angle on the firewall (so each aft oil cooler flange is connected), and a single adel clamp on the engine mount to secure the forward oil cooler bottom flange. This may require a short bracket to make the angle work to the adel clamp.
Recommend the firewall angle have some structure behind it, not just the SS firewall material. For you this would translate to the firewall angles being long enough so the ends attach to the cabin side vertical and slanted angles.
Carl