Front Leaf Spring bolt size?

I just purchased mine from a local hardware store, Mcfadden Dale hardware. I did the same thing, enlarged the hole to 5/8 (as I had a slant 6 car as well) and upgraded to calvert racing split mono leaf which needed the 5/8 and had a bushing for the 5/8.

You want a 5/8-18 X 4.5. Course thread would work as well. I just did fine thread because thats what the 7/16 bolt was previously. Also, grade 8 is your friend. Those leaf springs have some weight on them!

I bought a 5/8-18 X 5 as I plan on making my own traction bars and will need the extra length on the bolt but i'm 90% sure a 4.5 inch long bolt will work for stock application.

Hope this helps!