Custom framing a large piece

I know: it doesn't look like much.  But here's the point:  This painting is about 32' wide, and about 42" tall.
The frame is a very simple, straight profile with a white satin lacquer.  So what's so special about that?  Well, the moulding is 3 1/2" wide.  Those mitered corners have to be exactly at 45 degrees, else you're going to have gaps showing.  That's not something you can do with a $100 saw from Home Depot.  Our double-mitering saw is almost 40 years old, but it does a beautiful job.  And what a beautiful painting, done almost fifty years ago as a self-portrait.