I'm going to try and fire up a '68 International Travelall that I've had sitting around. The gas is probably at least 3 or 4 years old. It has two tanks. One has almost nothing in it and the other about 4-5 gallons.

Do I have to drain it or can I pour something in to mix and give it new life?

DRAIN IT.

XXXXX2222
A few years  back I built an SBC for a 56' F150 , the truck had been setting for several years and had nasty smelling fuel , but we  Lit the motor for a few seconds before going home that nite and the next day it cranked like it had no compression,turned out the varnishy gas had caused multiple intake valves to  stick , valves hit pistons The rest is history lesson learned , Old gas = goes to the Haz-mat now 

OK Roger that. To get it out of the field I think I will work with the nearly empty tank. I'll drain whatever I can get out (use an electric pump or a drill driven one) and then do I just pour some fresh gas in and then pump out and throw away like 1/2 gallon or so of it?

And is it OK to try getting it to run without tearing into the mech fuel pump and the carb?

BTW I am not trying to save the motor. Just need it to run for about 1000 miles then that's it.