Increasing gear range on old road bike

I have an 1974 Raleigh International that works great but I'm
considering going on a trip next summer that involves about 10000 ft.
of climbing and descending in 150 miles. I don't think I can hack it
with the gears I have now. It has 52/42 front and 13/17/19/22/25 in
the rear, all old Campy NR. I'm looking for low cost ways of
increasing the gear range. I really don't want to buy another bike,
but if this costs too much I'll have to consider it. Should I be
looking at a triple? Larger rear cogs? Any suggestions?