N8dawg: not sure if this would work for you on your 47 1st 3800 long bed but on my 48 I used 2 Pontiac Fiero tanks parallel to the driveshaft (I wanted to reserve the space behind the rear axle for my spare). The Fiero tanks are long and skinny, 10 gallons each. Not sure how readily available Fiero tanks still are? The tanks pics are in my December Gallery story below.
1948 Chevy PU

1948 5-window 3100 Chevy
327 V-8 TBI 700R4 Posi 3.73
"Old Blue"
In the Stovebolt Gallery
More pix on Flickr