efficiency

my 3607 best runs on plane at 16/17 knots 3200 rpm..

Most boats will get their best mileage just below hull speed. Since you may not want to go everywhere at 6 knots the next best speed will be just after she gets on plane. If you graph rpms vs speed it ought to be pretty easy to tell where that is. Of course since rpms can be converted fairly accurately to gph you can also find best fuel efficiency, or at least least painful.

4207 carver is a nice boat but it would be better with diesels and you can cruise all day without worry about your engines. Gas another story pushing a lot of weight. There is a lot out there with gas I would just keep searching.

As mentioned above the most efficient speed is just below hull speed which on that boat is likely to be 6.5 to 7 kts. Next best is fully on plane and usually with trim tabs down to keep the bow down

The absolute worst speed will be in between. Even just 1 to 1.5 kts above hull speed will typically increase fuel burn by almost 100%. Flow meters make that fact painfully clear. Try running half on plane, say 13kts, and you ll burn as much as you would at 18kts but going a lot slower, throw an obnoxiously monstrous wake and overload yr engines

I have a Carver 370 with two 454 Crusaders. I have a fuel flow meter on starboard engine and at 9 kts I burn about 10 GPH. However, if I use the trim tabs at 2500 rpms or 12kts, I only burn 16 GPH. Now if I go Wide Open at 19 Kts I burn 40 GPH.

you will have to check your owners manuel or contact carver, but I think you need to be close to 3000-3200 at cruise with a WOT at around 4200-4400 for 454s.

I think depending on intended use of a boat this size, you would be fine with gas 454s.

The 454 is one of the best marine engines ever made and parts are still quite reasonable and available.

You can rebuild or drop in a long block 454 for next to nothing when you consider the cost of rebuilding a diesel.

It really comes down to how much you will use the boat. If you are a recreational user with an intended use of around 100 hours the Gas 454's make sense in boats this size and smaller.

This is just a guess/estimate on my part, but I would expect a cruise speed of approx 15-17kts at around 34-36 gph and a hull speed of around 6-8 kts at 6-8 gph.....
Captnrontx is right on, i think these engines could push the boat close th 14-15 kts at 2500-2800 and you would be close to 24-26 gph....open the secondaries and open your wallet....

only way to know for sure is to run the boat.....there are lots of variables, props, bottom, weight of all the suff inside the boat.

Your 370 only hits 19 knots at WOT? Mine cruises at 3200 RPM 18/19 Knots and tops out at 28 knots WOT.

what RPM are you hit at W.O.T. I'd think you'd be in the high 20's for speed and hitting 4200 RPM . My 3607 hits 28 knots at 4200 with 22x21 props full of gas 250 gallons and 100 gallons of water, flat water and little wind. I'd guess you've got a dirty bottom, need a tune up or props need work (tuned).