this discussion has stuck with me;
I have always loved the 454SS trucks, and had thought I new a fair amount about them… but I am guessing some of that knowledge has faded with the mists of time;
I reached out to a buddy who has one, and what he told me might give the OP a sigh of relief;
He told me that the 1990 454SS trucks did indeed have unique seats, but thereafter they used the seats which had armrests - so a set from a Blazer (?ect) should be a good fit;
and not looking for specific seats now makes ones life easier.
I honestly thought you could order one of these with a bench (note I said order, I am mot implying they came that way);
so as a stop gap you could find nice correct year and colour door panels, and compliment them with a correct bench seat till you find the exact bucket seats you want.
IIRC the headrests on the bench seats came standard starting in 1992.
To me, bucket seats in a regular cab half ton, never made too much sense anyway.
I hope this helped!