I overexpose most of my negatives... so I guess what you've done isn't that strange! (you haven't underexposed by 1.5 stops!!) I'd use my normal developer, maybe dropping the time a little (say 10%) and print through the denser neg with longer exposure times.(posted 8159 days ago)As was mentioned, pick a methos, do a test using something less important!
Is "8x20" inches?