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Because of your criminal record at first you were considered to be inadmissible to Canada on the basis of criminality.But, after 5 years has passed since your sentence as been completed it is sometimes possible to be deemed to have been rehabilitated when you reach the border. However, there's no guarantee as it's up to the discretion of the immigration office.If you don't want to take a chance at the border the most prudent thing to do is to apply for individual rehabilitation. If this application is successful you will no longer have to be concerned about being turned away at the border.But, it takes over a year to get this processed.So, as you want to come to Canada before then it's sometimes possible to get what is called a Temporary Resident Permit (TRP). This is sometimes granted in cases like yours. Your case is strong because 31 years is a very long time. I would be shocked if you didn't get the TRP.Here is a link to the information about the TRP application:http://www.cic.gc.ca/english/information/inadmissibility/permits.aspHere is the link to the webpage that explains how rehabilitation works:http://www.cic.gc.ca/english/information/applications/guides/5312ETOC.asp
You can also wait and apply for the TRP at the border if you want to take that chance. As I've said your chances are excellent but there is still no guarantee.