The 383 has a smaller bore size so it doesn't breath as well. Also, most of the good performance parts are built for the larger bore size. A 4.375 bore size is very popular so there are a lot of ring choices as well as piston choices. A 400 block requires a 0.035 overbore to get to 4.375 so it is a very common size to use.

Check your local Craigslist for a 400 block. I see 400 blocks for sale on a regular basis where I live, not sure about your area.