I wouldn't go with that 260 HP. You have read between the lines. First off That's a 190 HP 2 piece rear main seal block not intended for marine use. it has the potential for 260 HP with induction. Mercruiser pre Vortecs are fitted with a 300 HP service head. the Vortecs are slightly more because they have better flow. I'll bet that 260 is a flat tappet cam too. Small valve spring and no compression :8.5:1" meaning the heads are 72CC combustion chambers. They won't work with a long duration Camshaft. These are the things you need to know. So know that you know that motor is inadequate for an 18 here's what I recommend. Seeing as how you had to assemble that any ways. Find a 350, 383 roller short block or best of all 400. go. up to 10.8:1 on Static CR use a Comp cam XM270HR to bring the Dynamic CR down to run on pump gas. Use a set of good flow aluminum heads to keep detonation down . If you run Iron heads you'll need to bring the Static Compression down some. The other thing you need to watch out for is the rear main seal. that will determine whether the fly wheel you have will fit. I'm building a 350 Reverse Rotation for my cruiser. I'm using the 350 Mag I took out of the Minx. Because it's RHR I get to choose from one flat tappet cam shaft if I don't have a custom core manufactured. What I have found is low compression is the biggest hinder to making a performance application. For instance the Mag is 9.1:1 Static CR the stock Mag Cam brought the Dynamic CR down to 8.0 .the cam has virtually no duration so I have to be very conservative on duration and valve timing when designing the new custom cam. To run on an Octane rating of 89 to 91 the DCR should be 7.5 to 8.5. You get a bonus .5 for aluminum heads. Aluminum isn't for weight it's to shed heat to prevent detonation in high compression engines.