Would,nt the B&M 360/904 flexplate be the same for 340/904?
The last time I checked, Mopar Performance had that weight kit but I would check to be sure it includes a template for your application. The 340's never came with a 904 so there is no factory off the shelf converter made for it. There may be a tranny builder that knows how to do the conversion but I don't know anyone personally. To answer the question, it has to do with internal or external balance, not what the cranks is made of. 318's came with cast and forged cranks but were both internally balanced. On the 340, all the forged ones were internal and the casts were all external but a cast crank engine can be converted to an internal balance. It takes some serious internal modifications though. In this case, I would try to find a 904 converter with the correct weights for a cast crank 340 or get a 727 and use a factory converter or the B&M flex plate with a zero balance converter.
