Does anyone know the fitting size for the rear brake line that goes to the proportioning valve for a e body? I'm installing one of the wilwood mopar 4 bolt master cylinder kits and the output port on the proportioning valve for the rear brakes is 3/8-24 and the rear brake line fitting is larger. Do they sell any adapter fittings to adapt the two sizes? This is sold as a mopar specific kit, yet the sizes don't work with the factory rear line.

at most good parts stores, there is usually a bin or rack with brake adapter fittings of all sorts, so you should be able to find something there.
i used to have some Edelman part numbers here, but i don't know what i did with them.
i'm thinking the larger size would be 7/16-14 [common for 1/4" line] or 1/2-20, still common size fittings used with 3/16" brake lines.

I'll have to keep looking around. Every parts store around me has everything but what I need

Why not call the place you bought the ill-fitting kit from.

The lines are the right size. They matched the original valve. It’s just the wilwood proportioning valve is too small. They only list the size that their fittings are, but not the factory size