Do not volunteer info at the border but on the other hand if they catch you lying your entry into Canada may be denied, They will probably ask you something like have you ever been arrested or been in trouble with the law where upon answering honestly will have you being sent inside for further questioning,its at this point you describe the nuts and bolts about your dismissed cases and put forward any documentation about your dismissed cases. at this point its solely on the border guards discretion as to whether you get to go to Montreal or not,dressing neatly,being courteous and polite will be to your favor but theres no guarantees.
If you know the border you are crossing at you could call them up and talk with an actual border guard and ask him what you need to bring with you as far as documentation .
Some reading on the issue=
http://www.ezbordercrossing.com/the-.../#.VYYd0FLP20Y
http://immigrationcanada.pro/criminal-issues/
best of luck.