we really, really, really need more info. first...you're wanting to ride cross country.
is this alone and self-contained? are you carrying camping gear? is this an
organized tour with your baggage carried in a vehicle? are you sticking to the
pavement, or do you expect to hit some dirt/gravel roads? when are you going?
what is the expected daily mileage?

second...you have some kind of bike. a road bike. what brand/model? frame size?
braze-ons? components? at the least, it'll need a complete overhaul - new cables,
new brake pads, repack or replace bearings, adjustments, new tires and tubes,
new chain......

you'll probably replace the crank with a triple, get new hubs and new freewheel.
may need to switch out the bottom bracket, too. replace the deraillers to handle
the gearing. now new shift levers if the old ones don't have enough throw. does
the bike have old weinmann center-pull brakes? will they stop a fully-loaded bike?
what's the widest tire you can fit in the frame or between the brakes? since you're
getting a wider hub, i assume you'll be getting a new wheelset in 700c rather than
the 27" it now has. will the brake pads adjust to the new rims? probably want a
new saddle, too.

replace all this stuff, then hope it all works together. of course, before you do all
this, find out if the frame fits you, and whether the chainstays are long enough
so that your heels don't hit the panniers.