The numbers come out to 43.9 minus 4 or 5 mph due to slippage, however, there is another factor that must be applied to the prop forumula. That is the ability of that much horsepower to push 12,000 pounds that fast. In other words, if the prop calculates out to 40 mph, for instance, the motors may not have the power to actually turn the props at 4200 RPM due to the weight.
My other formulas of weight, power and speed don't deal with props, they deal with the ability of a given power rating to push a hull of a certain weight. The info indicate you'll easily be in the 30-mph range, it would be interesting to look at the speed ratings from CC for the original boat (and I'll try to find that info). If I recall, the 427 was something like 38 MPH, which is pretty darn quick. 30 is quick for a cruiser. What is more important, however, is the midrange cruising performance. A wider prop is less prone to speed reduction with the loading of weight, such as fishing gear, but it is also a slower prop due to the addl drag.
All of this is great desktop coaching, but the real test is the in-the-water test, which is going to be lots of fun!
Keep us posted !
Regards, all the best,
Paul