thread length

I have added 18" Lorenzar wheels to my 1977 450sl. I had to use H/R 15mm spacers. They sent longer wheel bolts but they thread in about 3/4 of an inch. Is this safe?

the standard factory bolts for the "Bundt Cake" aluminum rims project 17mm inward from the inner face of the wheel, which is just a mm or so less than 3/4 inch, so you should be just about right, but when you first bolt one to the rear hubs, take the time to turn it very slowly to make sure there's no interference with the parking brake mechanism.