Yes, you are right about that.Stock axles for Dana were 23 or 35 spline, I think 8 3/4 were all 30 spline.
If you want to run a spool, I'm going to assume you want to run slicks. I would recommend aftermarket axles to use with a spool.
I would talk to Cass (Dr diff) and ask him about what he recommends for your situation and power level. He's a mopar guy and VERY highly respected here at b-bodys. He can also supply what ever you need.