I've replaced my mounts with RS mounts now and I've found the following:
I bought 30mm dia thick washers but they looked a bit small to support the large diameter of the curved washer that supports the rubber mount so I eventually went with 45mm. On fitting, I found the square hole in the engine mount is quite large so the larger washer is recommended for the 996.
The main bolts really do need to be 120mm long, perhaps the earlier 3.4 cars had shorter bolts? I bought the wrong bolts, 1.75 thread pitch rather than 1.5mm and 8.8 rather than 10.9 grade, I've ordered the correct bolts.
The nuts on the 12mm studs were very tight, after liberally dousing with releasing fluid, it still needed my 60cm bar on the socket and both nuts broke free with quite a crack. I thought the bolts had sheared but they were both ok. However the Porsche nuts are long and flanged so I reused them.
The standard Porsche mounts have a square locating point that fits the engine cross member, I did think about making up a square section to fill the space with a 12mm hole in it - another time perhaps?
The M8 bolts screw into captive nuts on the chassis, no nuts or washers are required. The Porsche bolts are flanged and I reused them as well.
My engine mounts seemed in quite good condition at 95k miles, after 15 years the main studs are no longer square in the mount but everything looked pretty solid.
All in all, it's a very straightforward job if you have the right bolts, when they arrive I'll find out whether there are any noticeable differences in the car.