If you're gonna build the frame up in that section, IMHO, just plate the outside of the framerail from the firewall body mount forward. You should have that whole area pretty much flat after you cut/grind off the IFS, and adding metal in the upright Y-axis of the box is bigger benefit there than adding in the X-axis.

With Isuzu's stout channel frame, you're buttressing a pretty strong base.

Randii