Step by step 4L80E, no floor modifications required in 68 GTO. Same swap done in a 67 chevelle in the super chevy link I posted, also no floor modifications. The transmissions that require floor mods are the manual 5-6 speeds ODs, not the 4L80E.
http://forums.maxperformanceinc.com/...d.php?t=823023
T400 durability, with OD, infinitely adjustability with electronic controls, or manual control if you want to shift the automatic manually.