Hello and thank you for coming to Just Answer and letting us help you with your question. Unfortunately, the Montana traffic laws require a CDL to operate a vehicle with a gross vehicle weight over 26,000 lbs on public roads. It doesn't matter whether you are operating the vehicle commercially or for personal use. Here is a link to a website on the rules for a Montana CDL.
CFR Section 396.12 (Code of Federal Regulations Section covering the Department of Transportation requirements) exempts privately-owned and used vehicles or single vehicle operations from the inspection and record-keeping requirements that apply to commercial vehicles. So, while you will need a CDL to operate your truck, you won't have to comply with other DOT standards. However, if you pass a port of entry while operating your vehicle, you will still have to stop and explain that you are not part of a commercial operation. The website for the DOT rules can be found here if you want to read it for yourself or look for more information. http://www.gpo.gov/fdsys/pkg/CFR-2010-title49-vol5/pdf/CFR-2010-title49-vol5-part396.pdf
I hope this answers your questions. Please let me know if you need further information about this topic.