Maybe I can help a little. I went to Prohm Kel from Siem Reap in March. It is on NH68 about 40 km north of Kralanh (which is on NH6). The road is under construction and it looks like it will eventually be as wide as an interstate highway in the USA. I think it is planned to be a major connecting highway to Thailand. I did not go that far north, but NH68 meets NH69 in Samraong. There is supposed to be an existing international checkpoint at the border with Thailand. I'll e-mail you the map that I found when I was researching Prohm Kel. From the border I think you would take Hwy214 to Surin. One option for you might be to hire a car in Siem Reap to take you to the border and hire Mr. Pirom (Pirom Guest House in Surin) to meet you there to take you to Surin. BTW, it took an hour to get from Siem Reap to Kralanh and a little less than two hours to get to Prohm Kel.
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