For the record those part numbers are backwards, found conflicting GM listings.
#7583551 are the small bumper skin end sealing washers, also used between spoilers and trunk lid. 5/32", ID 3/4", OD 1/8" thick. Also, see NPD washer link above.
#341092 are the larger bumper bracket sealing washers that also have the plain flat washers mounted against them. Both are 15/32" ID, 1-1/4" OD, thickness not mentioned but likely 1/8" also. Top 4 used only, lower 4 are outside of trunk.
79-81 models mention a larger diameter 1-3/4" washer still with 15/32" ID for the studs #3813432 but I'm not familiar so maybe a 79-81 owner can check. The sealing washers carry a new number but no sizes so may be similar.