Updated Buildcraft to 6.1.8 and got this crash on startup http://pastebin.com/4Yn7BBUd seems related to mekanism. Going to try reverting to 6.1.7 to see if it works, will updated afterwords.

That appears to be a pretty old version of Mekanism you have there

It's the recommended version on http://aidancbrady.com/mekanism/download/ 1.7.10-7.1.0.92, I'll try using the newest version

If you go slightly further down that page you'll find 7.1.1.125, try that

It seems to be working now after updating, sorry about that, would recommend updating the Recommend download link though

I did only fix it today to be fair to you - I'm sure Aidan will do so when he returns but I'm not able to, myself.

Updated recommended build!