more stuff to da stew
 

177, 250, 420, 671 or 871 blowers with moderate roller cams and roller rockers will get you well into to 70's with good props.
Keep the boost about 1 lb per 1000 rpm and you usually can run without intercoolers. As was said before aim for about 5250 rpm with brief shots at 6000 rpm. Keep the maximum boost about or a little more than 6 lbs and the blowers do not need to be tight only in good mechanical shape. There is a lot of used stuff available at moderate prices. I feel the 80's will require about 1000 HP across both motors and then you are in big block territory.

I have been wrong before.

With the new tech in cams and heads, I think a good 383 SBC would be comparable in Tq potential to a '90s BBC marine.

The quick solution would be a BBC swap. Solve the exhaust restriction for the SBC, and be cheaper in the long run.

I owned a similar boat, 82 Scarab II with twin 454/350HP Crusaders and Arneson ASD6's. Really no difference in the propping between Arneson and Kaama at equal HP.
I had the advantage of having Gary Garbrecht spend a whole day propping the boat with me so i know what worked and what did not, and I still own a set of the props so I checked.
17x24, I also ran a set of 16 1/2x24 round ears. No difference in speed, 68-70 depending on the water, these boats like it rougher to get the bow up, I was in salt water as well.
With 17 x 28's you have no where near the power to spin them. I presume you are at 300 or 350 hp stock. I was stock with 350hp and I would have weighed more with the big blocks. If you can get those things recupped to 24 you will be at LEAST a solid 60, maybe more.
The only other variable is the bottom of the boat, you have to have rocker, if it was a Wellcraft project boat like mine the bottom was blueprinted to take out the standard hook, but over the years it may have warped if it was sitting out of the water and not supported correctly.
Hope this helps, I really think it's just the props, you would need a MINIMUM of 600 HP to spin the 28's, that's a race boat pitch.
Even the 425hp Kaama package did not get anywhere near a 28 pitch
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I agree with Lee S.Fix the prop problem first then make sure the motors are running right first before you start sinking big bucks in to the boat to hop it up !!! Here are a few things to check: Look at the valve covers on the boat, those small blocks should have finned alum. valve covers on the engines,They rated at a very conservative 280 hp but they were putting out around 350 hp..If the boat has those engines you should have no trouble getting between 60 and 65 mph if you have the drives trimmed right!! That alone is an art in itself with those drives.. Dont give in to rushing things and spend a lot of money and be sorry later.


Good Luck


Jim
P.S. if you have any other questions P.M. me