Here's a layout drawing I did for the gas tank in the 53. To start with ordered a 73 blazer/jimmy 25 gal tank/sending unit from LMC catalog in all out of pocket cost is about 250.00 for tank/sending unit and strap set and it will have that factory stamped looking tank and not a square looking box hanging down along with being galvenized and not painted.
http://www.lmctruck.com/icatalog/bzc/full.asp?page=115
the measurements on this tank are 28 3/4 - 28 1/8 - 10 3/4
Haven't made mind up as of yet to do the filler inside the bed or under the tailgate with a sping license plate I like the spring license plate centered under the tailgate panel. Have got the frame boxed this weekend and waiting on 2/2.5 gal air tanks from VAIR should be hear by the weekend to complete the x-crossmember I made and ordered the transmission mount from Progressive. Here's a pic of the layout of the gas tank going in also will have more pics this weekend of frame boxed with the air tanks installed inside the x-member along with the gas tank mounted in the rear.