The way I would go to get what you want with the least frustration is get the roundhead mold designed for the heavy hooks (
http://store.do-itmolds.com/Round-Head-JigbrPRO-104brSz-38brHk-630-or-635brCollar-Barb_p_518.html). It says 3/0, but you can easily go to 4/0 without any mods... a little touch with the Dremel extending the eye cavity forward will allow it to cast 6/0 with no problems...you can tap the 6/0 hook in place with a hammer (lightly) or just use a good tight grip when pouring to get the best casting.
DO NOT TRY TO OPEN THE HOOK SLOT WITH A DREMEL... if you want 7/0, use a drill bit from the bottom. These tips should give you the results you are looking for.
NOTE:
Be careful not to get the Dremel too close to the part of the mold that contains the lead or you might booger that cavity... it can be repaired with JB, but it's best not to have the problem in the first place.
Welcome and happy casting.