I haven't removed the belt. I thought about trying to tension it somehow, but not sure how. I'll see if I can use a piece of small line to pull the two halves of the belt together between the pulleys and see if that provides enough tension.
Under the assumption that doesn't work though I'm happy to hear other suggestions
I have always just pushed the two sides of the belt tighter with my hand, that has always worked. The 1/4UNF bolts should not be very tight.
If that does not work, cut the belt, put the new belt around the pulley hold it tight, and the new belt should grip the pulley enough to loosen the 1/4UNC bolts.
If that fails, get a metal plate or a 7/16" spanner on one bolt, through the diagonally opposite bolt, hold the pulley via the spanner (wrench) through two opposite bolts, and undo another bolt with a 7/16" spanner (wrench).