Hardly use the hook size that Do-It recommends - down side to that is the mold will have to be modified. You will probably have to remove the center pin that you place the eye of the hook on the mold.
I use a Dremel unless it's big work - then I have a buddy that is a machinest. Like everybody says - do a little bit at a time. Once you remove this aligning pin (depending on which style and manuacturer of hooks you use) the hook can float around in the cavity when the mold is closed. This can cause mis-pours, eye of the hook can end up in the head of the jig, eye of the hook can ride to high causing a pitch in the angle of the hook. Don't think that would be too much of a problem with 60 degree hooks but you can really notice it with 90's. So, only remove enough of the aligning pin so you can place a hook in the cavity and get a good seal on your mold.
Another modification you might have to deal with is where the hook point rests in the mold. You will probably have to remove some metal there as well.
Mustad, Eagle Claw, and VMC have the O'Shaunghnessy style hook which is longer and has a smaller gap. I order mine from Stamina in the PS (it's a special order). Have you thought about the "Barbarian"? I think it was designed for trailers. I pour with them because they look like they should work. Owner has the Deep Throat.