Which Clutch Do I Need?

[sixtyfour]

Getting ready for the new motor, replacing the old 305 with a new 350. Anyway, was trying to order a clutch set up from JEGS and they were asking questions I didn't know the answers to. Oh, and this is for a 64 C10 with a 4 on the floor...

Tranny Spline Count, 18 or 10?

Clutch Size (Diameter)?

The dude on the other end of the line was saying that these old trucks didn't use a standard brand of tranny? Is this true? I'd like to know what tranny I have, may have to wait till the motor is out to find out for sure...