Here's some encouragement...............this won't be the last time you say "WHY DID I DO THAT!!!"

Realisticly it probably doesn't make much difference if you're building a typical hot rod. Meaning, you're going to have different size wheels (width & offset at least) and tires front to rear, so interchange isn't an issue. Most people don't do spares anymore with the advent of radial tires, and space consideration. Even if you do use a spare, there are dual pattern centers on some. Depending on which wheels you go for, many are made with dual patterns.

If it really means anything you can contact the folks you bought the fronts from and ask to do an exchange (probably wouldn't be the first time that's been asked), or have the rears redrilled by a competent machine shop to the Chev pattern.