Oy vay...
Steel (forged) cranks are internally balanced. Was the cast crank you swapped in from a 340 or a 318? 340 cast cranks were externally balanced, and required specially balanced convertors and harmonic balancers. 318 cranks, whether cast or forged, were all internally balanced. Depending on what supporting parts you used with the crank swap and whether you had the assembly re-balanced, you have a potentially messed up bottom end that will give you nothing but grief.
The B&M flexplate you need to use is dependent on the combination of parts that are used- i.e. an externally balanced crank with a neutral balance convertor, an internally balanced crank with a neutral balance convertor, an externally balanced crank with a weighted convertor, etc. etc. They're all different, so we need to know what you've actually got in order to know where the issue is.