1986 GMC C1500 shifts very early

hi, i bought this truck from a guy that told me that the tranny has a 1500 stall converter installed.. my problem is that when you take off the tranny shifts at about 10 mph into 2nd gear and shifts from 2nd to 3rd at about 20-25 mph. i dont know much about transmissions but this seems awful early.what is the correct converter for my truck? the motor is a 400 small block and doesnt have any hi perf mods that i know of. if i shift it manually the rpms get pretty high,( i dont have a tach).could there be a vacuum problem or is it the converter just not right for the truck. i drive it to school and around town (no racing). help please i hate having to shift it, thats why i found a truck with an auto trans. thanks