Is there such a thing as a 5000 stall converter that is streetable ?

Brooks James said

Would it be cheaper to have my BM 3500 conveter reworked to 5000 and a nitrous plate added, or buy new ?

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My opinion only. The problem with the old "marshmallow" converters was a result of loosening up a too-large converter to achieve stall speed. Results were sloppy low end, and slip at the top. You are FAR better off, with a nice tight eight inch 4500, than a reworked ten inch 4500.
If you want quite a bit more than the 3500, I would buy a new one, custom built specifically for your combo, and the nitrous.
My guess, it won't be cheap but it will be worth it. I've had a reworked cheap converter. Worst hipo part I've ever owned.