swap the rails over.
don't be dodgy...seriously
i've heard people saying "oh the seats fine, the other 3 bolts are in tight, just the 4th one doesn't line up, it'll be ok...the rail will twist like you wouldn't beleive in an accident..
i mean, rails get damaged if not stuffed with ALL bolts holding it in from an accident.
now im not here to be a safety inspector, but i want to give you that little extra thing to think about before you put something dodgy in the car that isn't exactly the safest thing out there.
i've said this before, what if there was to be an accident and u had 3 bolts holding it down (not saying u will leave it with 3 bolts because u want to cut and weld but u may be like, CBF) the person ends up dead because the seat decided to break off and they end up going at windscreen, side window, pillars, dash, whatever. say they were to die, you end up in gaol buddy (not to mention the mercy of regret)
but yeah, i'd say swap rails or get the rail done by a pro.