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Welcome, from TX. Very good find! Is your adjustment all the way out on pump? May slip the next size shorter and wider belt over pulley an see if it fits the channel any better.
Do not pry against the reservoir! It will permanently distort and may begin to leak.
If it's got headers the aft bracket is probably gone.New quality belt?
Here's an example from an '87 R10 that uses an R4 compressor, adapted to function with headers.The bracket really stabilizes the P/S pump, making the mount rigid.
Am I missing something here? I don't see any pictures at all! I just get big blank posts with little x's on them! By the description, it sounds like you have a nice truck! Welcome from Northeastern AZ BTW!