The adapter (usually the adapter is integrated into the mount that connects to the cross member) will be the easy part. Depending on your mechanics skills or luck it could be an easy swap. If you can find a 203 that was originally mated to a 400 it'll slide right together. If not the output splines of the 400 could be different than the input on the 203. No big deal if they aren't the same. Because the 203s were made to mate up to different trannies they were produced with different input gears. You can go to a gear parts store and get the different input gear for your 203. Count the splines on your tranny and the gear store should have it.
Depending on what you are using the truck for you may want to have the tranny output shaft changed to match the T/C. As far as weak points in the drive line are concerned the 203 is very strong, if you change the input gear to one that has fewer splines you are adding a weak point. If you have the output shaft on the tranny changed to a larger one with more splines you are adding a stronger part to your tansmission. If you do a lot of off roading go with the strongest parts. If you are into snow removal I have no idea what kind of pressure is placed onto the driveline. I would imagine that it would be similar so again you may want to go with the strongest parts.
I purchased a 400/205 combo for my Blazer but wanted to keep the 208 that was already in it. I swaped the 400 in place of a 350. You'll have to find the adapter that comes from a 400/203 pair or the dealer has some pricy parts. The tranny shop that did my swap was kind of shady and I was the one doing most of the homework and leg work chasing parts. I ended up having the stronger gear put into the T/C rather than having the tranny tore down again. If you have a problem locating the adapter give me a shout, I MIGHT be able to get you one. Not promising anything but I know the owner of an off road shop and he has a small junk yard.