I've got one bolted in to a car with a bare engine compartment, but I'm not exactly sure of what measurements you're looking for.
The pic is a little misleading..I needed 3 hands to take a pic of how I measured. I held the tape against the flat vertical surface of the firewall just above the tunnel (below the lowest rib), then dropped the plumb bob down to the back side of the tube.That helps a ton, I went fire wall (not the ribs) to the front of oil pan and have about 1/8" of clearance. So hopefully the plumbob went down across the ribs and that will be about another 1/8" and I think I can alter the oil pan as well to make it get in there!!