Frank, there shouldn't be any problem in using data for a 158 JHP with a 158 JSP. In fact, most bullet manuals (e.g., Speer, Hornady, etc.) use the same data for both types within a given weight in most cases. And as long as you're not pushing maximum pressures, you can usually substitute one brand of JHP or JSP. That is, if the data you have uses a 158 Hornady JHP, you should have no problem using that data with a Speer 158 JSP in loads below maximum.
If you want to use HP-38, go to the Hodgdon web site and get the data there: http://www.hodgdon.com/data/pistol/index.php
Using a 158 grain Hornady XTP/HP, they show 6.9 grains as maximum in .357 Magnum; 4.6 grains as max in .38 Special +P, and 4.3 grains in .38 Special.
A couple of notes. First, the above loads are considered maximum. It is usually a good idea to start 10% lower and see how the loads perform. Half a grain below max is often a good starting place with loads like these. That said, there are obviously no concerns if you used 4.3 grains of HP-38 with your 158 JSP in a modern, quality .38 Special revolver rated for +P, and certainly not in a .357 Magnum revolver, since such a load is well below +P and magnum levels.
Second, HP-38 is the same powder as Winchester 231. There are slight differences in the loading data due to normal lot-to-lot (and load-to-load) variations in powders. But again, if you're below maximum, you can usually use W231 data with HP-38.
Third, a piece of gratuitous advice. If you don't already have one, buy -- and read! -- a good, comprehensive reloading manual, like the latest from Speer, Sierra, or Hornady. Not just the loading data, but the sections on reloading basics.
If you want to use HP-38, go to the Hodgdon web site and get the data there: http://www.hodgdon.com/data/pistol/index.php
Using a 158 grain Hornady XTP/HP, they show 6.9 grains as maximum in .357 Magnum; 4.6 grains as max in .38 Special +P, and 4.3 grains in .38 Special.
A couple of notes. First, the above loads are considered maximum. It is usually a good idea to start 10% lower and see how the loads perform. Half a grain below max is often a good starting place with loads like these. That said, there are obviously no concerns if you used 4.3 grains of HP-38 with your 158 JSP in a modern, quality .38 Special revolver rated for +P, and certainly not in a .357 Magnum revolver, since such a load is well below +P and magnum levels.
Second, HP-38 is the same powder as Winchester 231. There are slight differences in the loading data due to normal lot-to-lot (and load-to-load) variations in powders. But again, if you're below maximum, you can usually use W231 data with HP-38.
Third, a piece of gratuitous advice. If you don't already have one, buy -- and read! -- a good, comprehensive reloading manual, like the latest from Speer, Sierra, or Hornady. Not just the loading data, but the sections on reloading basics.