best 340 tranny??

If you're just going to install a used transmission, I'd get a 727. If you're going to rebuild one, either trans will work fine but you'll need to do a few upgrades to the 904 to handle the extra HP. You can also reuse your driveshaft and yoke for the V8 904. If you haven't done so already, you'll need to do a rear axle assy and brake upgrade too.