Try the FMVSS 571.301 fed regs for fuel systems
http://www.ecfr.gov/cgi-bin/text-idx?SID=f3d3f17a0c7663af0acc75f5bc9aedf3&node=49:6.1.2.3.38.2.7.64&rgn=div8..
And if your commercial or you sell the truck with the home made aux fuel tank to a commercial operator read this from FMCSA
http://www.fmcsa.dot.gov/regulations/title49/section/393.67 and
http://www.fmcsa.dot.gov/regulations/title49/section/393.65.
And as others say contact your state size and weights/commercial division for their input on how your aux fuel tanks needs to comply....if any.
