1970 390 flexplate

they are different

I had two 68/69 service replacement blocksm one NOS and one used, and both had the correct 68/69 deck height, were balanced correct for the 68/69 model year and had the correct pistons, rods and crankshaft for 68/69. The 70 service replacement block was a different block and used the taller block for all ther 70 390 and 401s but should have the correct internals for whatever years it replaced.

What may be your case however is that someone unknowingly used a later service replacement block that was not correct for your year at some time in the car's life and swapped the earlier balancer and flex plate without realizing they are different than the ones that go with the earlier engines. For all you know, you could have a 401 lurking under the hood and the only way to tell is to pull it out and drop the pan to see what crankshaft and rods are actually in the block.