into position when the fenders won't open up any farther. Car is a '70 Road Runner. Need 1/4" to 3/8" more. Fenders have paint, but the valence will be handed off to be painted later in the week. Thanks for any help.

I think the only thing that you can do at this point is bow it in the middle.

loosen up all the front bolts in the headlite bucket/fender flange support areas to allow the lower sections to spread open for the lower valamce install, if you can't fit the lower valance your going to have issues with the grille install as well

This may be a bit late but are the fenders original?
Is the valence original?
is the hood original?

Assuming yes it should go back together correctly
If no you should have mocked up the assembly to ensure fitment.

Loosening fender attach hardware should allow you to open the span between the fenders to allow the valence to fit, but will also open the gap on the hood. If this becomes an issue you may have to keep the upper bolt snug and rotate the lower fender outward to fit the valence.

Door/fender/hood fitment took about a week for me to be satisfied with gaps and panel alignment. It can be a frustrating task, every time you move one item it creates a change in another location.

Good luck.

Doors, Fenders, A/G Hood & Hinges, and Valence(I have 2), are not original to the car. Really happy with the gaps so far. Last night I took some measurements. Both Valences measure the same length. Then I hooked my finger on the upper fender valence mount point and pulled. Both fenders did the same thing, as I pulled forward(away from the car, I could see the valence interface open up. I've been just pulling laterally with no success. It is as if the fender is uncurling. It should be good. Thanks everyone.