46,
you should be able to do a good ring job. Those "babbitt" motors give you the ability to adjust the fit for wear. You cannot do this with the insert type. The tricky part will be trying to keep track of the shims. I suggest getting a new set from Chevs of the 40s, and reshim when you put it together, by the method in the manual. The only problem that you could run in to is if the babbitt shows signs of dropouts, otherwise go for it. I did my motor in place and it turned out really nice.
Oh, and those iron pistons don't usually show too much wear either!
