My truck has a 700 r4 trans W/O removing it, how can I tell if the torque converter has 27 or 30 splines?

The truck is a 1989 Chevrolet full size, 2 wheel drive, 4 speed, overdrive automatic transmission, with a 350 motor. The torque converter is bad and I am told that there are two different spline options, a 27 spline and a 30 spline. How can I tell which one I need without removing the transmission? Has anyone had a high milage tranmission go out after changing the fluid?