Considering replacing my Holley carb with the Sniper EFI on a 502. With elbow top headers, where can I install the O2 sensors so they won't get wet. Is it even possible?

If you travel over to the Sniper group on FB, you'll see that there have been issues. Considering that everything is built into the unit and needs to be serviced by Holley if it breaks, your boating time can be ruined if you're got to wait for Holley to get you a replacement after looking at it.

There have been a couple here that went that route successfully. I had thought about it and went with carbs. Much easier to deal with any issues that pop-up. And if something tanks, it's pretty easy to fix with parts that are quick and easy to find.

You can use O2 sensors to help with carb tuning. I mounted O2 sensors in my tailpipes right after the collector at the 10 o'clock position. I also use a spacer that keeps them dry. I've run them about twelve hours so far without issues. I monitor using an Innovate DLG-1.

That's just my take.

Here's the location on mine. It's in the tail pipe right after the collector about 2:00

No issues with my sniper going on 3 years now.

I have them on small blower motors (177's on 454's), I'm not using them for ignition control. They have been easy to use and set up, have not really done any tuning with them.
I made spacers for my GIL/MERC manifolds and are using an Innovate Motorsports spacer for the O2 sensor.

I should ad, when you are doing your budget, don't forget the cost of lines, fittings (supply and return), filters and pumps. All of that adds a significant cost to the project.

I use these cheap O2 extenders. They were consistent with using no extender on the dyno.
https://www.walmart.com/ip/Spree-Car...E&gclsrc=aw.ds

Sniper is a gamble. Go to there facebook or tuning forums and a lot of people have issues but some love them. I personally would not run in marine application because you will be loosing O2 sensors regularly because of water revision unless running extremely mild cam and dry tail pipes. There is a reason Mercury does not run O2's. You could get your base tune and then turn down O2 sensor closed loop but you are loosing most of the advantage of sniper EFI. I know there are a lot guys that will disagree with me but when your stranded with your sniper on the water you will wish you still had your carbs.

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I sure wish someone produced spacers like that, I could use a a set. Hate to drill the tailpipe and weld a bung in them.