I'm not aware of any "correct" bearings that will do that. I do remember some talk of making spacers to use the 340 crank in a 360 block, but I don't know if anyone ever actually made them.
hmmm I wonder how hard it would be to make them... (or have them made)
Anyway yes the 340 and even 360 offer many more piston choices then the 318 and like you said I can also use factory 340 pistons that you guys are selling cheap because you are overboreing your 340's to clean them up ect..
