I don't know what kind of resto you are doing, but for my DD resto all I did was the following:
Got some scathing hot water in the kitchen sink and poured a little bit of every kind of soap and cleaner (fabric safe of course) that my wife owns and mixed it around. Then I dropped the seat belts in it a set at a time, let it sit for about 2 hours, then went back and rinsed them out. I did this 2 times, then set them out to dry.
For the chrome and the plastic I used a sponge and some elbow grease. Then once all the gunk was off it I rubbed in some buffing compound and that seemed to really make them pop.
Like I said, i dont know if you are going for a concourse resto, but for a DD resto this was more than adequate and it blends in just fine with my all new seats and carpet... I couldnt justify $1,200 or even $400 for a job you can do over a weekend for minimal funds and effort.