Yes another Gas tank question :P

[72okme2u]

I have a 72 swb k10 on a 73 and up frame. Poor ol truck has been patched to death and nothing quit right. My question is has anyone tried mounting another model of tank under bed?? i know there are plenty of relocation kits for a 21 gal. but my theory (right or wrong) is stock can hold 18ish gal. i would like more than 3 gal. more if going thru all that trouble. After doing a lil research found out a model of bronco/F series that holds 33gal would fit after some fabrication. Has anyone ever done anything like this or other model ideas??
Thanks From the new guy