Disclaimer: I can't be responsible for you screwing up your crap
I've heard of people using 3M Window Weld to fill in trans and motor mounts but I prefer a 2 part liquid RTV Urethane. I used a 94A shore urethane that I purchased from McMaster-Carr for around $28. It is a premeasured 1 pound kit that is just more than enough to do the tranny mount.
Here is a picture of the urethane...
This first picture is the stock mount.
The second picture shows where I removed the center rubber triangle. For this I used an exacto saw. The reason I removed this is to give more space for the urethane. I left the outer two rubber pieces and just encapsulated them. I also removed some bits of rubber that were left over from the original molding.
Now make sure the mount is super CLEAN. Use degreaser to completely clean the mount. It's also a good idea to take some sandpaper and rough up the inside of the mount.
Next, take duct tape and seal one side of the mount as shown below. Make sure you leave room for the bolts to go back in.
Side that you will pour into...
Make sure you follow the instructions on mixing. It is best to use all of it at once as it is premeasured and has to be within a 1% ratio.
First prop the mount on something so that it is level and pour slowly.
Here is a picture of the mount after filling with urethane.
The urethane takes either time or heat to cure. At room temp. (77 degrees) it takes it 2 days to be 80-90% cured and 100% in 7 days. If you don't have that much time like me, you can also place it in the oven at 165-190 degrees for about 10 hours. This was per instructions from a rep. at Forsch Polymer.
Good luck and happy hardening