Don't fly the 38 much except for maybe in the DA for practice and the MA's for jabo runs... but this is what I've found.
Since all the guns are in the nose not a whole lot of conversion choices are needed. The only one you'll really have to mess with is the 20mm. I have mine set out a bit farther to keep it a little closer in line with the 50's as it travels. To short and it drops to fast, to long and it travels slightly above the 50's.
There are many posts on how 20's travel through the air. Trying to find that perfect trajectory to meet up with your 50's at your chosen conversion point (or WHERE YOU NORMALLY TAKE YOUR SHOTS) is not easy.
Use this thought process for setting the 38's 50's then test it out using the .target command and you'll find I'm right.
As the bullets leave the guns from the nose of the plane they are in the shape of the nose which is maybe 3.5 feet wide (?)
The convergence is set to meet at one point like the tip of a cone. After they meet they extend out in the other direction creating another cone. The farther they travel the larger the cone on the other end. Well you only started with 3.5 feet so twice the convergence set would end up the same size as the nose again.
Because of this I like to set my 50's at about 175 to 200. At 200 they hit like a laser. At 400 they hit like a shotgun.
Any farther out and they spread into a larger cone but you have to keep this in mind.
This is where setting your 20's comes in handy. Try to match the 50's dropping with the single 20 for the best convergence. Basically all the bullets will stay in the same path and your hit sprites will all be in a circle I like to call "The Cone of Silence"
I use 175 for 50's (=D300 nose cone) because my shots are normally taken a little closer and 400 for the 20's