38 total is a big block number and is little too much for a small block. I would stick to 34-36 total.

The two sb MP distributors I have purchased were set up with 24 degrees in the distributor to go along with 12 initial (set up for a stock engine). Depending on the cam you are using 15 initial or even more may give you the best idle and off idle throttle response. This will require you to limit the mecahical in the distributor. The easiest way is to get the Mallory tuning kit (#29014) also sold under a mopar number. It has spacers to set up the stops and comes with an assortment of springs. As you limit the amount of advance in the distributor you need to go to heavier springs to keep it coming in at the same rpm. When you get down around 20 degrees in the distributor two orange springs or one orange and one brown work well fro a street driven car (all in around 2500 rpm).