Welcome to the forum. You will find that many, many others have had the same question before. Using the search feature turns up threads like the following:
http://forums.pelicanparts.com/showthread.php?threadid=165762&highlight=modify+911t
http://forums.pelicanparts.com/showthread.php?threadid=223097&highlight=911t+mods
http://forums.pelicanparts.com/showthread.php?threadid=227195&highlight=horsepower+911t
There are more threads in the archives. But to give you a specific answer, the 911T is a difficult starting point for a high-performance engine because virtually everything will need to be changed. For example, the case is magnesium and will probably require a line bore and perhaps other machine work like fixing the through-bolt holes or machining the spigots flat. Also, depending on whether your case has squirters, you may want those to be installed, and the oil bypass modification is definitely recommended anytime the case is apart.
The 1970 911T crank was not counterweighted. Some racers prefer this crank because of the lower rotational mass, but it is noisier and the bearings will wear faster than a counterweighted version.
The cast-iron cylinders are not the best choice for a high-performance application. Even 84mm Birals should probably only be bored to 85, so if you are looking for 86, you should consider an aftermarket cylinder.
10.5:1 will require twin plugging the heads with the associated distributor modification, or crank-fire direct ignition.
External oil cooling will definitely be required, so factor that into your plans. Pelican sells the required parts and Elephant Racing has an excellent web site as well.
Finally, the carburetors will need to be modified to cope with increased power, which is difficult to do with Zeniths.
http://forums.pelicanparts.com/showthread.php?threadid=260809&highlight=zenith+weber
I hope this is helpful to you.