The LS2 doesn't protrude into the tunnel area. You should only have to modify the tunnel if your particular subrame pushes certain transmissions too high.
I hope that made sense, it's hard to explain. We only had a fab a small part to mate the tunnel to the new firewall, but it didn't raise the tunnel at all.
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"A ship in port is safe, but that's not what ships are built for."
See Bad Penny run the cones:
https://www.youtube.com/watch?v=8GUPPIX-92U
1971 Chevelle Wagon - Roadster Shop Chassis ProCharged Shafiroff LS and lots of yada yada
1968 Camaro - Project Track Rat - 440 RHS LS