Hi Trent:
My boat has a fitting similar to what you describe, which was for the old Volvo engine. The previous owner had installed a new engine and had placed a through-hull and seacock in the middle bilge compartment which I've also removed and then glassed over the hole. In my effort to reduce the total number of through-hulls, I now have a 3/4-inch through hull and seacock under the sink for all saltwater needs (toilet and engine) plus another 3/4-inch through hull and seacock for the sink drain. The remaining through-hull and seacock is 1 1/2 inch and is located in the heads for the toilet/holding tank discharge. This arrangement has worked a treat, plus I have two fewer holes below the waterline. If you want to use the engine water pump as an emergency pump you can tee-off the intake and lead a hose into the bilge. This hose can also be led into a bucket of fresh water + anti-freeze, if you plan to winterise the engine. Make sure your connections are tight and double-clamped and you use isolating valves where necessary. In the Accommodation section, under "General plumbing+holding tank" you'll find a note with several photos. The last one illustrates my set up under the sink. If you need more, let me know. Also, at this point, I'm installing two new bilge pumps but I'm not ready to post anything yet.
Nicholas
Moments of Clarity #334