How high are you going to spin it? If it was mine and it was going above 6500, I'd get an SFI plate, but that's me...I don't like the idea of my drivetrain grenading.
I'd definitely upgrade it since you have a big block. What size converter do you need it for? I ask cause I have an extra like new heavy duty one I'd sell cheap. It had an sfi sticker on it but the guy I got it from painted over it so I don't know how old it is. Just that it's alot heavier duty than stock.
I would go with a SFI approved flexplate if you are building a "performance base street driver" and have not installed the engine/trans yet....its good insurance like stated above.