To determine which transmission would work best for your truck you must 1st consider what the rear differential gear ratio is, the rear tire diameter is and how the truck will be used.
The problem with a car manual transmission is the first gear ratio isn't typically correct for getting a heavy load moving. The granny gear these trucks came with were there for a reason.
But if you are seldom pulling a load, or hauling a heavy load, a passenger car manual transmission will work as Wrenchbender describes above.
I have a Muncie 4 speed M-21 passenger car transmission in my truck, a 3.73 rear gear and 33" dia. rear tires. It is great on the highway, but you have to slip the clutch a bit to get it moving. Ideally a steeper rear gear (like a 4.11) would be ideal, used with an overdrive transmission, also as Wrenchbender discussed. This gives you a good overall gear ratio for pulling / hauling loads, but good highway mpg also.
This gear ratio calculator will help in sorting out what will work best: http://www.4lo.com/calc/geartable.php
Another thing to consider is an overdrive unit for your existing transmission.
Hope that helps