don't concern yourself with such trivial matters dan. a mild build with reasonable gears won't benefit much from an OD gear.
unless you want to go manual transmission (you don't, won't or can't) the amount of slice and dice needed to shoehorn an automatic one in is a tall order. the price of admission is high no matter the route you take and if you can't do most of that work yourself it can be a nonstarter financially.
but if the weather sucks and you want to read up on it search 518, 500, 42RH or 46RH there's plenty of threads here to lead you down the rabbit hole.
