Same here, slot size affect total, springs affect rate. Swapping weights is a old Chevy thing because the shape of the weight also affects the curve.
However, your second question on whether they should be tight. I always think that a well lubed, properly clearanced weight will be more predictable than a floppy one, but I haven't had to do that on a Ford. So if it's something you can make more precise, can't hurt if it doesn't bind, but someone like Faron has a lot more experience on how, or whether its worth it.