That large nut in the center holds the steering wheel boss on. If you loosen (don't remove) that nut you will need a puller to get it off. You could also bolt the steering wheel back on, pull it up really hard and evenly and at the same time have someone (you trust) smack the shaft with a BFH and a brass punch. If you can get that loose remove the nut and you will see the splines underneath. Count them and measure the OD of them. A steering boss like this
Moto-Lita, the Classic British Steering Wheel is readily available for common splines. As Larry said looks like a GM.
There are some options on 14" wheels here
Moto-Lita, the Classic British Steering Wheel
I am a Moto-Lita dealer so if you need anything from their you could let me know or contact them directly.