Welcome Aboard ! First of all if it was my boat I would be looking at an intake conversion with a carb, and would not go with the fuel injection simply because a carb is a dumb slave to air flow and needs no electronics to work, thus, it's more reliable on a boat. However, I greatly respect a personal preference and will do what I can to assist. We have another member of The Forum who did a fuel injection on his big block Fords, and here is a link you might find interesting.http://www.network54.com/Forum/424840/thread/1329238664/%2A++My+fuel+injected+468+cubic+inch+FE+rebuild+for+38+Commander+LADY+DI
On the Q intake manifolds, there is a lot to consider if you are swapping so let me know, we do have a thread with a lot of photos and info on that.
Regarding a location to pick up a reading for an oxygen sensor, I think your idea of a spacer will work just fine but it will have to be an inch or so thick and be drilled so it won't interrupt any water or exhaust flow. On the standard marine system there just is not a good place to find a spot for an oxygen sensor. Of course it can be done but not easily.
Here is a flow diagram and a photo to help understand what you have to work with.
As a sidenote, the 600 cfm Edelbrock #1409 marine carb is way more than enough for the small block motor at 4000 or 4500 rpm. A performance intake with that carb would do considerably more than the fuel injection would, in my opinion, because the stock intake is limited and it's a proprietary item on the Q motors, not easily swappable because of how it's made. In the end, for your use I would stick with the stock intake and carb, and if the carb is shot then I'd go with the #1409 and go boating.
Keep us posted, let us know how you're doing, and send photos too.