Fe drain through house-garage foundation

I wish building departments had a weekend question number. I am hoping I can get an answer so I can complete this work this weekend and get it inspected next week.
I am relocating one drain line and adding another drain line from under a raised foundation house into an attached garage on a slab foundation. The drain lines are cast iron with no-hub fittings. The garage slab is about 13 inches below the floor of the house. The first five inches of this is a foundation wall, the rest is 5/8 drywall over wood. The drain through the wall is a 2 inch combo with a clean out on the strait through and the bend making the turn up the wall. One drain line is for a washing machine and the other is for a utility sink.
The existing drain lines went through a notch in the slab. I had planned on doing the same until I remembered reading about the five inches of concrete being necessary by code to keep spilled gasoline from flowing into or under the house. I have no idea where I read this, is this correct?
I can refill the notch around my pipe with concrete. Is this sufficient?
Will the rubber in no-hub fittings be a problem with this barrier?
Do I need to wrap the cast iron in pipe tape to protect it from the concrete?
In the worst case I can put the combo through the 4 by 6 on top of the foundation (rim joist?) and then use two 45s to drop below joists and plumbing. This also has the advantage of eliminating the heavy tools and work of cutting the notch in the foundation.
Thanks for any help or suggestions.
Richard Kaiser

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