Hi all,
I have found that over time seat belt webbing does get stiff, frayed, nicked,
grubby, and just downright questionable as to weather or not in will do its
job when needed.  New belts are quite an expense.  What I have been doing is
cutting out the old webbing, clean and lubricate the latches, etc.  And then
buy new webbing at a local upholstery supply house, along with some heavy
polyester upholstery thread, and then just sew some new belts on, using the
old ones as a guide (for how to route the webbing through latches and reels,
and stitching pattern for strength).  For this material you don't need an
industrial sewing machine, a home machine with a needle for heavy material
will work great.  In fact you'll probably become so encouraged that you'll
start making straps and belts for other purposes.

Cheers,
Kevin Dawson