I converted from a 6 cyl/automatic to a V8/3 spd, then to a 4 speed then to a T5 over a number of years. Never had to trim or modify the tunnel in my '65 at all. The spacer that goes between the bell housing and the T5 put the shifter in the correct position. All I had to do was to remove the shift tower because the sifter stub (that the shifter bolts to) stuck up. I took it off, spun the trans into position and then reinstalled it using high temp silicone sealant. I also used the T5 shift handle but had it heated and straightened to resemble the shape of the stock handle. I have 3 speed and a 4 speed shift handles laying aroung and will someday install one of them...maybe....
67/68's have larger tunnels than 65/66. The only debate out there is weather or not you really "NEED" to modify the tunnel on a 65/66 and even then, the debate only applies to the installation of Tremec 3550's and TKO's.
There are no chassis modifications required (Except to enlarge the shifter hole slightly) when installing a T5
Now T56's, or Richmond 6 spds are a different story.