Andy -
I want to confirm my answer to your question. I installed the interior trim in my '54. I was correct in my original post ...
The #8 screws with #6 heads from the Truck Shop matched the original trim screws exactly. The "interior screw kit" from the Truck Shop did not match the originals. The "interior kit" screws were #8 with #8 heads. The only "kit" that I used were the screws for the lower door seal retainers.
FWIW - Headlight bezels also use #8 w/#6 heads.
Click for picture:LEFT - Original trim screw
CENTER - #8 with #6 head
RIGHT - #8 with #8 head