The B20 is better but from the headgasket down it is more or less
interchangeable. The B20 heads have different ports, smaller valvestems,
MUCH better valvestem seals, and bigger valves. There are at least 3 and
maybe 4 different B20 heads. (A/B, E, early F and later F) and maybe even
a late B head. In addition to head thickness and combustion chamber depth,
compression ratio is also varied by headgasket thickness.
The B20s have at least 2 different oil pumps, both of which have higher
capacity than the B18. Then the 8-bolt B20 uses different crank, rods and
pistons, although the main bearings are the same size. The 8-bolt stuff
is compatible with the B21, etc.

And of course the B20 has the letters "VOLVO B20" cast into the side of the
engine block at the front while the B18 has the letters "B18" cast into the
side of the block at the back.

While you can bore a B18 out to 2 liters and get a good engine, the combustion
chamber is too small and hangs over the open cylinder, so it is not optimum.
--
George Downs Bartlesville, Heart of the USA!