[54belair]

Im glad you mentioned the slow fill issue------------ (I didnt want to bring it up)

If I see what I think I see (?), that could be your fill issue--------------In one of the pics, looking into the filler neck, it appears that your extension is slipped over the existing (on the tank) cap flange. I was wondering if the flange aprox. 6 inches down would cause backsplash when filling. Sounds like it might , if indeed the original neck flange is still there.
Ive talked to several guys at car shows who have done this type of tank install---and havent heard of filling issues (with the camaro tank and neck).

Anyway, its a thought-----------