A few things I use all the time
Dead blow hammer (HF has the orange ones and the heads are full of sand) great when you need the force but don't want to bang up your work.
Brass Drift punches: Great for driving things like bearing races when you don't want to chance gouging or scratching the bore.
bearing and seal driver kits: I use these all the time. How many of us ruined seals because it wasn't straight when going in?
honing stones: For me these are one of the most used tools I use when assembling or rebuilding. whether it is to debur as surface or remove high spots from a gasket surface.
file set and file cleaning brush: These are common, but don't forget a file cleaning brush. It doesn't take much to clog a file, especially on aluminum.
emery cloth: a must for polishing shafts and cleaning parts.
feeler gages: a must for adjusting valve lash, points etc..
Torque wrenches: If you plan on doing an engine rebuild these are a must have! (inch and Foot pounds)
*Calipers and micrometers: At minimum I would get a set of calipers for measuring parts, If you are not familiar with using them buy a digital set. These are rather important when doing a restore! Being a lot of these old GT's are hard to get parts for in many cases you will have to measure bearings, bushings and seal to find or make an alternative part.
These are just a few of the things I myself use all the time.