"But, you have found load data for the bullet and powder you want to use, except that it uses a .38 Special case instead of a .357 Magnum case. Because the .357 Magnum case is just a little longer than a .38 Special case, AND because the .357 Magnum is loaded to much higher pressure than the .38 Special, it is clear that the powder can be safely used in the .357 Magnum case with a slight adjustment. At least, it can be used for loads that are no more powerful than the .38 Special loads in the data."
Yes, good point, and absolutely correct.
I took the query, however, to mean that he wanted to use it for .357 Magnum to develop .357 Magnum power/velocity levels, not to duplicate .38 Special ballistics.
For that task, it can't be done safely.
It's also a pretty good bet that if you can't find information for a particular bullet/cartridge/powder combination from the powder manufacturer, they already did the testing and found that, for whatever reason, it was NOT a good combination or was even unsafe.
"The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza
Baby Jesus cries when the fat redneck doesn't have military-grade firepower.