stupid disc brake compatibity question

[spookylynx]

Alright, like it says, I have a question that is probably dumb, and I think I know the answer to but I'm going to ask anyway just to be sure.

I'm running CPP drop spindles and am going to buy a 6 lug disc conversion kit.

Are the calipers in the different kits ( CPP, McGauty's, LMC, etc..) all the same size? and interchangeable.

For example, McGauty's says you must use their spindles with their brake kit. I think this really just a marketing ploy. I think they are all pretty much '73 calipers right?