The cabs in 1958 and 1960 are the same as other models D100-D900, W100-W500.
The 1959 cab has a 1958 flat floor and the 1959 style dash. This cab was used on D700-D900 and W300, W500 only. A real pain to find if the original cab needs replacement and you want the truck to be correct. The standard internal step cab will fit if you don't care about being 100% original, and you could always weld in 1958 floor patches to replace the steps if you wanted. But it is an odd an annoying, yet interesting detail.
Generally speaking the 315 in the 1958s failed and got replaced with a later engine. The 318 is a much more robust engine and is often still intact. Lots of people will tell you they have a 1 ton 4wd as they think 8 lugs is a 1 ton.
factory FH6 5.87 trucks, which means they do about 40 as a realistic speed.
A V8 4.88 truck with 9.00x16s will cruise at 65 without trouble.
The W100-W200 trucks are based on the same frame as a D100 and use dana 44 front axles. The W200 only has a couple more leaves in the springs and a different rear axle otherwise it is basically the same as a 7 1/2 foot bed W100
They are toys compared to the W300 which is based on the D500 frame, has Dana 70 axles front and rear, and uses a 9 foot bed.
Little Mo, something you find in the Dodge W series sales literature. There are not that many out there But there are enough little mos out there that if you wanted to convert that one to a pickup it wouldn't remove the last complete example from the country.
Here is an article with some little mo info, scroll down to see period pictures: http://www.coachbuilt.com/bui/b/blitz/blitz.htm
Here is one in a museum: http://home.roadrunner.com/~homeville/1959.htm
Check the W300frame over carefully. The sheet metal is all the same as a 1960 D500 so even if it was junk somewhere out there is a good western $1,200 body. The frame is unique and would have to be repaired if damaged or you might as well start over. The engine swap would just require some parts from a 1959-60 D400 or so truck. That is assuming you want to keep it looking stock. Keep in mind that while it might never matter I think a factory V8 with the correct V8 may be more desirable down the road.
The original tranny is a 420 with the big truck input shaft but closer to light duty ratios.