26 inch ECP in a 68 Dart

No, no trimming. Not for mine anyway. And that "sheet metal" you're pointing to is your frame rail.
My bad I meant the metal on top of my finger not what I'm actually pointing to but thanks it's nice to know I won't have to cut any talk to Champion today and they gave the measurement overall to the top of the cap on the filler neck as 23 3/4 looks like I am in business thanks to you I will wait to purchase though because they give you 60 days guaranteed to cool