This is a pic of the US MAG 15in sbp with a 255/60/15 M/T ET STREET S/S with only a 3in 6 leaf lift spring plenty of room between tire and spring iffy on fender lip this is on a 64cuda
Make sure your extension has the proper hole/holes in it. If you have a rear pump it will need a two hole extension. I know a 64 904 has a rear pump and should have two pickup holes in the extension
I went with vision 15x7 rims with a 0 offset and 4in back space with M/T 255/60/15 which are approximately 27in tall and 10in wide which has 1in between tire and spring and it's tight between tire and fender lip
I do have 6 leaf springs with a 3in lift and stock shackles and stock leaf spring...
Thanks toolmanmike I'm so close.
I just have not been able to find a 15/7 with at least a 4 plus backspace and 0 offset other than the Cragers without having a custom set made