Others with more experience than I have will chime in on possible solutions. A few choices come to mind: you can change the ring and pinion in the rear end, buy an overdrive unit or even something simple as larger diameter tires. Here's a website that you can play with to see what rpm you are turning at 55 once you measure your tire diameter. BTW, 55 is pretty fast for the design of the truck back then. I'd guess 45 was 'top' speed back then.
http://www.advanced-ev.com/Calculators/TireSize/