You should not have to mod the tunnel for a 4l60 they are the same as a 700r case and thousands reside in trifives.

I Also might add you guys running these OD trans behind even a stock LS1 be careful with a stock 3.5 drive shaft. I twisted a brand new one off at 135 mph about a foot behind the trans. I'm lucky to be here. In my case my front angle on the trans was a bit much which probably contributed to the failure I raised the tail an inch. I went back with a 4 inch and thicker wall basically nascar truck specs drive shaft. There have been many failures on the dyno with ls swaps in these older cars. Especially when they start making big horse power. The dyno has to run the car up to top of 3rd gear. I was just below the top of 3rd pulling strong when mine let go....