Hello and welcome.
I been a dodge guy forever and will help the best I can. It's good to have a dodge to be different but the downfall is that the aftermarket has not been very kind to them in reproducing parts. There are no aftermarket patch panels that I know of that are made for a '48 or earlier dodge trucks. I hope somebody can prove me wrong here...
You will probably either have to find a donor truck and cut the pieces you need or make them yourself. If you give fabrication a try you may love it. Best of luck with whatever you do.
As far as tranny's go. Im not aware of any small v8 that will bolt up to the '48 transmission. Id hit a junkyard and get a donor 318/904 combo. You can usually get them cheap or free. They are super reliable as me and a friend have had 318's go well into the 300k mile mark. Or you can get you a late 70's diplomat,aspen,or volare and grab the front clip with the 318 and weld it all up. This will give you the motor mounts already there along with p/s and disc brakes, rob the column while you are there to. I didnt know if you wanted to retain straight axle or not. Straight's look cool but the IFS of the '70's mopars will drive/steer/stop way better.
There are lots of ways you can go here and as stated good luck with what you choose.
