I can sum up the Canadian tax rate for >$80k in one word, HIGH. Not going to tell you how much I make, but my tax rate is above 30%. Also, just like in the US, if you work a lot of OT expect to be taxed to death, but you get a portion of it back on your return. As for the per diem, I have no idea if they will extend it for the entire time. It would be a good question to pose during negotiations with them. As stated by BeerCourt talk to a tax pro before accepting!! You would much rather know ahead of time what you are getting in to then when it is too late.
As a side bar, you will also be eligible for health care after 90 days. You will need your work visa with an expiry date longer than 6 months from when you apply for your health card. It is a pretty good idea to do this ASAP once the 90 days has passed, otherwise if you have a health problem you will pay out of pocket or through your private insurance assuming you have it.
Cheers,
Jim Rodgers