I've never brewed a Mild, so I thought I should give it a go with a proven recipe. Here are Jamil's proportions of specialty grains for a 10 gallon batch:

Crystal 60: 1 lb
Crystal 120: 12 oz
Pale Chocolate Malt (200 L): 8 oz
Black Patent: 4 oz

The closest I could find to Pale Chocolate Malt @ STLW&B was some Belgian Chocolate Malt at 350 L.

Any suggestions on how to modify the specialty grain bill?

I have plenty of pale chocolate if you need it. Last time I check anyway...