I am replacing the belts on my 77 280z and noticed that the Alternator belt was loose but not squeaking and there was no more room on the adjustment bracket to tighten it up more. Can I use a shorter belt just as long as it is the same with ?

Yeah, that's what it needs. While you're at parts store you may want to find out if the new shorter belt is the proper part # for your year. If it's not, the PO may have changed harmonic balancers or even engines. The balancers in different years had different diameter sheaves.

Mark in Portland