I just replaced the mount in my '65 with a T5 conversion. I removed the nuts from the bottom of the mount, the bolts that attach the mount to the transmission and the bolts that attach the crossmember to the "frame". The crossmember won't come out because of the exhaust pipes underneath but it dropped a little. Then I jacked up the transmission as high as it would go and was able to pry the mount out with a long screwdriver. I noticed that there was about 5/16" of thread showing on the studs on those 2 mount-to-crossmember studs with the nuts in place so I cut off about 1/4" of the studs on the new mount. With the shortened studs the new one went in a lot easier than the old one came out.
I'm sure I've read that the T5 sits higher than the original transmissions so you shouldn't have any problem with the C4.
I just had mine off on my 67 and I'm not sure you can get to the bolts that hold the mount to the transmission with the crossmember in the way.
I have a 200, so I don't have exhaust pipes in the way, but it seems to me that it was a lot easier to just throw a piece of wood on the jack and hold up the transmission and take the crossmember off than it would be to try and get those two bolts out.
The bolts in question, seen here in pink are going to be a pain since they come up from the bottom. The crossmember bolts on the other hand are a dream and come right off.
I agree with Magnus, although it's been awhile since I tried to do that on my '66. I say "tried" because I saw no way to get to those two bolts, and my main concern was the fact that the engine and transmission shifted backwards when I took the mount bolts out. The car may have been higher in the front at the time. I managed to wedge it back forward enough to get things back together.