the GM marine motors us the "Chevrolet" bolt pattern. the pattern is the same for the 2.5 (153 cubic inch 4-banger), 3.0 (181 cubic 4-banger)inch), 3.7 (224 cubic in 4-banger), 4.1 (250 cubic inch 6-banger), 4.6 (292 cubic inch 6-banger), 3.8 (229 cubic inch V6), 4.3 (252 cubic inch V6), all SBC V8's and all BBC V8's as well as the Duramax diesels.
so yes, while you can bolt an SBC where the 4.3 V6 was, the drive ratio is different along with the mount locations.