Pretty sure that 1.25mm can't be true anywhere in the top of the bores... maybe the skirts but not up top.
Now maybe on a bike engine you can go thinner than on a car engine, but anything less than 3mm on the thrust side would seriously creep me out...
But you know what, maybe this is one of those "try and see" things, where you spend some money, learn some lessons, and end up going with the whole 400 block in the end after all ;-)