If the belt is loose and you can't tighten it any further, you need a shorter belt. That being said, it is always a good idea to get the right parts. I wouldn't try to add a tensioner; it never had one to begin with, so trying to add one will only complicate things and be more jimmy rigging.
'65 Coupe, 302, Autolite 4100, C4, 2.80
'72 Grande, 351C, Motorcraft 2100, FMX, 2.75