How do you safely eliminate 2nd gas tank in 93 F150 truck?

remove the tank and close the line off,but leave the vent line open on the one tank your going to use,tie the two vent lines together,this will keep the system working like it should,i did this before and if done right it will work well,good luck.

I would chain it to a tree or something, load the family in the bed of the truck for weight, light a cigar, and drive like a bat out of hell to the end of that chain.

Ps. While it's empty of course. Safety first ;)

go to a mecanic and hav them do it !