Look at the passenger footwell, inside wall, you can see a cut-out at the top of the aluminium heat shield towards the rear, that's where our engine harness went across.
We actually ran wiring for front stuff (lights, horn, thermo etc) through the top chassis tubes. If you do, make sure you mark the tubes with tape and write on it, "warning wiring" while building it so you don't accidentilly drill into the wiring.