Game set match...back to the drawing board. Keeps popping off, needs serious adjusting.
Those tensioning pulleys (the two medium-sized ones without blades) should have springs, or maybe one spring, to keep the belt tight. They should be moveable (or one moveable).
Perhaps when the breakdown happened a spring broke and is now gone.
Check for places on the deck and the pulleys where springs could be attached. A lever with a hole in it is one possibility.
Springs also tend to cause paint wear marks on older decks. Check that out.
The B348-7 mower seems to have more information about it but it is very similar to the B348-6.
cmoringinstar01, a member here, appears to have a manual for your mower. You may want to contact him if he is a still a member. You might be able to get a copy.
http://www.orangetractortalks.com/forums/showthread.php?t=4223