Ken Waters, in his 1970s groundbreaking article that created the three action strengths for the .45-70, rated the H&R Safari break-open .45-70 as equal to the new 1873 Springfield "trapdoor" .45-70.
This is the "Group 1 - Weak Rifles" category.
That said, you can probably safely use loads intended for the 1873 Springfield, but no hotter.
The article is in the book, "Handloader's Digest No. 7," which is copyrighted 1975. Though the article is old, the information is still applicable.
Waters did not list any 500 gr. jacketed bullet loads in the Group 1 section. I've found numerous references to 500 gr. lead, lubricated bullets suggested for the 1873 Springfield, but no jacketed bullets.
You cannot just swap bullets with this data; the lead bullets produce far less friction and resulting pressure than jacketed bullets.
You will probably have to contact the powder manufacturers and ask them for a Group 1 load with 500 gr. jacketed bullets.
Your rifle sounds ferocious. Shooting 500 gr. bullets from a 14-inch barrel should get you a Christmas card from your local surgeons and orthopedic specialists.
But your rifle is hardly the worst "thumper" I've seen. Some years ago, at the Salt Lake City (Utah) gun show, I saw a Ruger No. 1 in .458 Winchester Magnum for sale. The barrel was cut to 16 inches and the stock and forearm thinned. It weighed about 7 pounds. A rifle like that can break or fracture bones!
I left it where it was. I never quite got the hang of walking sideways.
Have fun with that beast -- and may God have mercy on you!
"And lo, did I see an ugly cat. Smoke. Brimstone. Holes in parchment. And this ugly cat was much amused." --- The Prophesies of Gatodamus (1503 - 1566)