Magnum gurus needed

Some other input: I bought a 5.9 rebuilt short block with everything, similar to yours. Had everything checked at a machine shop before assembly.
Once started, it smoked(oil) So had to pull it out. Had it completely disassembled. Turns out the rings were installed upside down. They are thinner than the LA's. Second machine shop said that has happened other times.
As someone else mentioned, have EVERYTHING checked.
If you are going with the Magnum belt set up there is picture of my 75 Swinger. Look under "photo garages" page 9 about half way down listed
as Triumphman as a reference. Good luck with it all.