Spindle diff. between disc and drum brakes?

[ChadGiles70]

I have a 70 blazer that I'm converting to disc brakes using an axle out of a late 70's truck. I'm having issues with the inner brake pads not fitting between the dust plate and the rotor for one. Also, the caliper on the driver side is hitting the steering arm! Any thoughts from you guys? But I was noticing in other threads that people were talking about the spindles being changed as well, What are the differences between them to make it that you have to change them? I got a little frustrated last night so any help will be great!