This from Wika:
The 198 was introduced in the North American market for model year 1970 as a more powerful base-model engine than the previous 170 engine. The increased displacement gave improved vehicle performance and lower manufacturing cost, for it was achieved with the tall RG block also used with the 225 engine by installing a crankshaft with 3.64 in (92.5 mm) stroke and connecting rods 7.006 in (178.0 mm) long, for an actual displacement of 198.3 cu in (3.2 L). Manufacturing costs were reduced by eliminating using two different blocks for the two different available sizes of slant-6 engine. The 198 engine was available through the 1974 model year.
So bottom line, they would not need 2 different blocks. Just a new crank and rods. They could then charge you extra for the larger 225 over a base 198.