When you say your cylinder has been modified, i assume you mean a .357 reamer has been used to re-chamber it to .357 Magnum. In that case, it is not safe for .357 Magnum ammunition.
You assume correctly...
I agree that without question, a .38/44 N-frame revolver will handle any .38 Special Plus-P ammunition on the market. So-called Plus-P-Plus at pressure levels at or under 24,000 PSI should be safe, but I wonder about some of the loading data I see. For the record, Phil Sharpe listed one .38/44 load (158 grain lead bullet/1100 FPS) at 31,000 CPU, but if his numbers are correct (as I believe they would be) I would work up to that VERY carefully!
Now I will confuse things a bit more. Prior to about 1922 (I don't have my reference book handy) S&W didn't heat-treat cylinders. They would buy quality bar stock, machine it into cylinders, fit it to a gun, and then finish the part (blue, nickel or whatever). During the 19th and early 20th centuries when black powder was used you couldn't get enough of it into a revolver cartridge to risk blowing up a top quality Smith & Wesson or Colt. As smokeless powder came into the picture this changed, because it was quite possible to over charge a cartridge case that had been designed for black powder. That's still true today.
So in or around 1922 going forward, both Smith & Wesson and Colt started using a chrome-moly alloy that could be heat treated for some, but not all cylinders. In particular, cylinders in K-frame .38 Special and .32-20 cylinders were heat treated, and when it came along, cylinders for the N-frame .38/44 were too. I'll refer to them as "standard heat treated."
When experiments that led to the .357 Magnum came along it was soon clear that some proposed loads were well in excess of what the .38/44 was supposed to withstand. Smith & Wesson then purached a special grade of steel that could be double heat-treated for extra strength. Cylinders made for the test-bed .38/44 revolvers were fitted with cylinders made from this material, and as an additional precausion the case head was countersunk into the back of the chamber, and chambers were burnished to smooth them for easier case extraction. When the .357 Magnum was introduced in 1935 this cylinder in both design and material specifications was made standard for both it and other Magnum revolvers as they were introduced.