Easy way that I've raised tunnels in the past is to slice it along the floor, add the height I need for the bigger tranny with a strip of 18 ga. and weld it all back together. The trimming and welding is time consuming, but it makes for a good looking tunnel. I've also used the aluminum ones (Comp. Eng. in Florida sells a nice one) and installed them with Dzus fasteners to facilitate quick tranny and clutch changes.