Depending on the guitar your winding for, Determines what is standard for that type of guitar.
50mm will be fine for gibson type guitars.
Fender type guitars are more F spaced.
Here are some parts to look at to get ideas.
Guitar Pickup Part by Type - Mojotone.com
STEWMAC.COM : Guitar Electronics, Pickups, Pots, Switches, Caps, Shielding and Parts at
Kits are the best way to get started.
Mojo and Stewmac both have kits.
Just pick the pickup type.
Good Luck, and Welcome to the Forum.