Well, I'm in the middle of what will be my 2nd AG batch--something similar to Bass if I get it right. If a recipe calls for 1 lb of 55L crystal malt and all I have is 80 and 40, what happens if I mix 80 and 40?
Primary Fermenter 1: Beer
Primary Fermenter 2: Empty
Secondary Fermenter: Empty
Bottled: ESB Stout
You get 60L.
Use just a bit more of the 40 than you do 80 and I think you will be fine. 80 degree will lend a bit more roasty flavor but I'm thinking it won't be very noticeable.
EDIT: After running the numbers through ProMash, if you want the same color you will need to use very little of the 80 degree. Maybe just use the 40 degree if you have enough?