Transmission swap
The 4L60E fit in the same tunnel space at the 2004R, but I did have to modify my rear motor/trans mount. No cutting on the floor pan itself was required for either.
The most recent overdrive transmission swap i've done was to remove the Extreme Automatic's Stage 2 - 2004R and put a re-geared, Stage 2 - 4L60E from Performabuilt Transmissions. These transmissions are rated at 700hp & 900hp respectively they are compact and durable when fitted with aftermarket parts.
The 4L60E fit in the same tunnel space at the 2004R, but I did have to modify my rear motor/trans mount. No cutting on the floor pan itself was required for either.