i've seen trucks out there with no motors and a "Serial Number" tag on the drivers door jam or under the hood that was either missing all together, or so worn down you couldn't read it.
The non-stock plate from the DMV outta have whatever number is on the title.. so long as those two numbers match, you're good to go regarding the legal use and sale of the vehicle.
Original engine numbers don't seem to tell you much except for the year of the motor and it's size, if you're looking for the number that would provide the most information for your truck, it would be the mfg plate (see Jeff Nelsons earlier post for location).
From the numbers on that plate, you can determine with a reasonable level of accuracy, what year the truck is, where it was made, what kind of truck it is (1/2 ton, 3/4 ton, etc) and in the 1933 thru 1953 trucks, it'll tell you what month it was made, too.
I've seen some restore jobs where this plate was removed to paint, and never made it back on, tho.
