Those small-block Ford marine engines from the 70's were something of a strange beast. They all had basically the same heads as the 351W so the exterior dimensions were the same and all the accessories were the same for the OMC 175/190/235hp (and the equivalent models of Mercruisers and V-P's that used those engines). The OMC 175 and 190 were 302's and both had 2-barrel Holley carbs but the 190 had a different camshaft and used the same firing order as an OMC 235 which was a 351W. The 235 had a 4-barrel Holley carb. If anyone knows more about these engines, please post it.
As far as "converting" a 175 to a 190, probably the simplest way to pick up the missing HP would be to go with an aftermarket four-barrel carb and intake manifold (a manifold and carb from a 235 would bolt right on). In the real world it's hard to say whether you'd notice the difference though.