340 spun bearing rebuild.

If you get a "competent" machine shop, it will mean nothing as far as the timing chain. A good shop can align hone or bore and either not move the crank bore at all or very, very little. the proper way is to remove material from the mating surfaces of the main caps, reinstall them and then align bore it so that the bore removes more material from the caps and barely skims the block. This will rarely move the center line of the crank bore.