I picked up a 25' Eliminator Daytona with a 572 and a mooneyham 10-71. It just has the external balanced pulley adapter on the crank. I was under the impression that this was a drag race type setup, and not for endurance application with long duration between tear downs. I was looking at buying the Romac balancer and putting it on, but my question is it will it help out being that it was not balanced with the rotating assembly, (is the balancer a big part of the balancing procedure?) and it would be beneficial to bolt one on?
You can have your new balancer balanced to match the balance of your existing hub. Make sure that the balancer you purchase is the same, either internally or externally balanced. I highly recommend that you get the part manufactured from 4340 with the dual keyways. It takes a lot of power to drive your blower and a single keyway and hubs made from soft or brittle material are not up to the task.
As far as, “is the balancer a big part of the balancing procedure?” is concerned, the hub or balancer is a big part of the balance if your crankshaft is externally balanced because the counterweights manufactured into your crankshaft do not have enough mass in them to counter balance the mass of the pistons, rods, rings, oil allowance etc., so weight is added to the hub or balancer and flywheel to compensate. You can see by the attached pictures, the weight added to the external balance hub is substantial. Just make sure that you match the type of balance of your new balancer to that of your existing hub, internal or external. Then a good shop can balance your new balancer to match that of your existing hub. Not being balanced with the assembly will make no difference.
Regarding, “would be beneficial to bolt one on?” There are many studies both ways. Some say that the blower belt dampens out harmonics and that a balancer is not needed. Some say that adding a balancer will not only increase crank and bearing life, it will increase horsepower as well. Or maybe ‘not rob power” is a better way to say that. Case in point, the Chevy 302 used an 8-inch balancer because Chevy felt that the benefit of reduced harmonics made more power than reducing the mass by using the 6-1/4 inch balancer.
Bottom line, I don’t think that it will hurt anything as long as you don’t mismatch the balance type and you don’t split the hub or spin it on the crank by shearing the keyway. You might see increased bearing life. You will not feel any difference in power or speed.
You might also consider one with an SFI 18.1 safety rating. Even if you don't need it, you are going to be stressing it driving your blower. Your boat probably has an enclosed engine, but I like mine totally exposed. If that belt ever comes off, it will come off with a lot of energy and will run through my boat like the Texas Chainsaw Massacre. Just my $.02