I know of several places to pick up parts for Ford/Chevy, but I haven't been able to find one for Dodge. Ideally I'd like to keep the outside with the same look so I'm not too interested in body parts at this time. I'd like to "modernize" everything under the car so that it can be used as a daily driver.
No one is going to just have parts for these trucks, they actually take some skill hunting parts down and doing research unlike a chevy you can build the whole truck out of a magazine, I know these truck very well 55-60 D100's I own a 57 currently which is nearly identical. the best bet for modernizing the drive train would be a early 90's Dakota frame swap, which gives you mount options for small block mopar, 5x4.5 lugs that your truck comes with stock ( as apposed to the later 6 lug dakotas. front IFS, and a heavy duty 9.25 rear, now im more of a fan of 8.25 but a 9.25 will handle most big block applications. Avoid doing a Volare IFS swap I see that gets recommended alot on the internet. I "attempted it and was very disappointed with the outcome, also parts are beginning to be impossible to find for Volare swaps. Mustang 2 IFS swaps work amazing but they are super expensive, If you happen to be looking for body parts http://www.mar-k.com has alot of bed parts I have seen a few companies on ebay sell the rear fenders in steel and fiberglass since those same fenders were used from 54-82. hope this helps out some?