if travel by air to Laos the overstay is a problem, you will be denied boarding. As a general rule it is no problem to pay the THB1,000.- at airport. Robert Lagas summed it up nicely and correctly.
you could get a SETV in Vientiane/Laos or you get a 30day extension first, than either try Kanchanaburi by yourself or Laos. You could also fly OUT/ IN to get another 30days. Like fly to Phnom Penh or Siam Reap and spend a few days there.