Kev,
Yesterday I was talking to a driver who had been in Canada for 4 months, he had been nicked in the UK for having a bag of weed, he had to do the American Embassy thing and be de criminalised, this got him a B1/B2 Visa which cost him about a grand I think, I watched a TV show recently where they were in various prisons, one bloke got 7yrs for a couple of ounces of weed, so they take it quite seriously, if the bloke I spoke to got in with a drug conviction, then it looks promising for you, so good luck mate 
Are you sure you want to do this though? You know you’re going to miss the A13 