I've run different length belts on a 1972 4/4, but I did so as the crank pulley was a bit smaller and the much modified engine reved higher, limiter set to 6800, but probably safe to 7500.
I'm surprised if the 725 is too slack/long, a 715 won't go on as it's only 10 mm shorter (number is the length). With mine, to get the shorter belt on, one had to swing the adjuster right out of the way by removing the bolt that secures it to the alternator. That way the belt would easily go on, but if you left all the bolts in place, no way would it have gone one.
Pretty sure I've also fitted a tight belt by using a screw driver a bit like a bicycle derailleur but I do have my bush mechanic moments.
