I'd advise you have a chat with any of the many helpful people at the Bury track. Someone will be able to check the temp of your motor after a run and you may find its not as hot as you think.
Also, as mentioned, racing on a track where you're accelerating or braking, all the time, on high traction does get the temps up much quicker than just bashing around.
The only things I'd suggest you check as your concerned are,
- Is the mesh to tight?
- If you take the pinion off does the transmission turn over freely by hand. If it doesn't the diffs could be shimmed too tight or a bearing somewhere could have seized.
If you check out all that and its nice and free I'm sure you won't have any issues.
Hope that helps.