From Thailand, normally you do 1-day visits, leave your passport at the border & be back before sundown. There may be some longer tours available by some tour operators, though.
But as far as I know, no vehicles into Myanmar. There have been some individual reports about succeeding with even this, but only after HUGE paperwork & effort, and still not riding/driving all the way thru the country, have to freight your vehicle and fly thru some parts.
Interesting news about the superhighway, though... hope its not just some distant dream. Arent all those dictators, military juntas, police states and other oppressive governments such a pain in the ass, for their own people naturally, but for overland travellers too???