The B19 has exactly the same displacement as the B20. But the B19 is an underbored B21, so it has a cross flow head. It was intended only for the 300 series, but may have been available on the 240 in some markets as an alternative to the B21 (just like 10 years later when there was a 2L B200 engine as alternative for the B230). Perhaps your father was offered the choice between a 1975 with B20 and a 1976 with B19 (which would already have been available in the last few months of 1975). The 2 engines can not be swapped without modification, the B19 sits at an angle and the inlet manifold is on the wrong side. Mounts are also different. Can be done and I have seen photos, but not a simple swap.

But you may want to investigate piston compatibility, as someone suggested. Or have a look at KG Trimming, they have a lot of nice goodies.