Cashiers check, Bill of sale(with ID and witness), and then take car with you and wait for title.
What if he takes the money and runs? Likely, no. Possible, yes. Then the possibility exists the finance company finds the car, repos it, and the OP is screwed. Yes, the seller would face criminal and civil charges but the OP is still screwed. I wouldn't do it unless the finance company has a local office where the transaction could be done in person.
I did this once with a co-worker. We went down to her bank (that held the title) and I paid the bank cash for what she owed on the vehicle and we signed a bill of sale so I could go to DMV and transfer ownership. Bank sent me the title in a couple weeks.
I did this also but I was the seller. Just met at the credit union that held the loan. Paid it in full and got proof that the title was free and clear. The buyer then took the info to the DMV and got the title transferred. Was a pretty painless process.