I took mine out and soaked them in a bucket of hot soapy water (with laundry detergent.) Then went at them with a good stiff scrub brush, rinse and repeat if needed, and hang outside in the sun to dry.
First I removed all the buckles from the belts. (you get to learn all the ways to twist and turn the parts to get them off). Take lots of pictures because they are tricky to put back together the right way! Then I used Dawn dish soap. It cut the grease much much better than laundry soap. I filled a five gallon bucket with soapy water, let them soak a few hours, used my hands to swish, squeeze, plunge, etc. Dumped the grimy water, repeated this about 6 or 7 buckets of clean water worth. Then I hung them up to dry with clothes pins and hangers.
It was amazing how much cleaner they were. And then with the buckles apart, you can clean, sand, and paint any rusted parts and polish the chrome pieces. It was a couple of afternoon's worth of work, but well worth it!
I used to be indecisive. Now I'm not sure. I feel like I am diagonally parked in a parallel universe. 1968 400 convertible (Scarlet) 1976 T/A - 455 LE (No Burt) 1976 T/A New baby, starting full restoration. 1968 350 - 4 speed 'vert - 400 clone (the Beast!) 1968 350 convertible - Wife's car now- 400 clone (Aleutian Blue) (Blue Angel) 2008 Durango - DD 2008 GXP - New one from NH is AWESOME! 2017 Durango Citadel - Modern is nice! HEMI is amazing! 1998 Silverado Z71 - Father-daughter project 1968 400 coupe - R/A clone (Blue Pearl) (sold) 1967 326 convertible - Sold 1980 T/A SE Bandit - Sold