Those are 500/525 castings, with less plenum volume they offer no real power advantage over a 502 mag intake with simple mods and bored out TB other than possibly a peak hp rpm gain from slightly shorter runner, the fact you would have to come up with rails, throttle body and the rest of the missing pieces, I wouldn't waste my money.
You can PM me on the modded tb and injector stuff, I don't want to get banned for skirting the forum rules which I may already be doing even mentioning the work I do although I am at least a paying member.
as far as the fuel system, all of the merc stuff is strange inverted flair, to upgrade it you need to change EVERYTHING except stock water separator filter housing.
You would buy a holley 750 efi pump or a a-a1000, put it on a relay triggered by the original efi relay as it was never intended to supply uch more than a stock 6, 7 amp pump. You would tap rail where stock fitting goes in, buy a efi regulator, a auxiliary fuel cooler , a 10 micron filter after everything and then -8 and -6 hoses, hose ends, adapters etc. However, IF your hp goal is 550, as long as your stock stuff is in good working order, it will be fine and save you quite alot of money.
Flame arrestor: https://gaffrigperformance.com/102-EFI-Flame-Arrestors
Its a beautiful, high flowing piece but you will also need to buy (4) 35$ studs from them or find some gen V FA/tb studs as its impossible to mount correctly without them.
O2 bung: if using stock cast manifolds, you can buy or make a 02 plate to sandwich in riser but it is very hard and cobbled looking to move riser up 1" and still have silent choice connected to it.
A nice alternative I just did on a stock riser in my machine shop was install THIS piece: https://howellefi.com/product/adapto...or-marine-use/
You could run (1) o2 on the 2/4/6/8 side and be fine, IF you buy a holley terminator, it will only run one anyways.
02 longevity: That bung in a riser along with a innovate 3729 to help keep moisture off 02 sensor would make it work the best but I would prob STILL get it dialed in then remove and plug it.

Good aluminum heads along with intake/tb mods and cam will get most 502s to the 600 to 620 hp range from the 550/560 range with ok cam and iron heads, the heads are typically worth 50 to 60 hp as long as you take advantage of the extra flow with right size cam and usually aftermarket exhaust, Smitty

When I put the Gen V flame arrestors on my Gen VI 502 MPIs, I bought threaded rod from McMaster and cut a couple inches off. I think the final length was about 8 inches each. 316 Stainless for $4 each.

https://www.mcmaster.com/90575A276/

Be careful: mcmaster carrs shipping $ was great for long time. Now it super sucks.

Awesome again thank you for this information. I am currently running the Eddie Marine Thunder aluminum manifolds and cast risers but I think I am going to swap out the riser so to the mandrel pipe style risers maybe I can have Eddie Marine add O2 bungs.

I spoke with Holley and I cant run Terminator X as it is not Coastguard approved and as moisture resistant like HP due to no encapsulated ECU on X like HP.

Fueling I have heard horror stories about the "cool fuel" and getting to it once the engine is in the boat looks to be a REAL PITA so I may prefer to spend the extra money and upgrade the system to a good quality pump and regulator. Sorry for the 50 questions but do you have a recommendation for a fuel cooler I assume remaining water cooled?

I will PM you about the TB & injectors.

I've noticed that too. This order wasn't terrible.

Really I got stuff the following day from them. That sucks!

They're still fast -- but not as cheap (but what is, these days?)

Ive done that with the gen V FAs too, I HIGHLY advise against doing it with the Gaffrig though, as in I dont see it being possible since I tried several ways and had problems. Because the gaffrig is not as ridgid as a Gen V and the threaded rods have to hold tb in place AND hold Fa on, what happens is it squeezes the Mesh on FA enough to jam into the throttle arm once you actually have it tight enough. The factory gen V studs (which is exactly what gaffrig is selling for 35$ each) have a locating shoulder that goes into the Gen % TB, on a gen 6 TB, it has to have the 4 holes spot faced in a mill for the countersunk tb fasteners (no big deal). So in the course of trying to run one with threaded rod on dyno, I almost had catastrophic destruction of a customers engine. The gaffrig also uses 4 little 1/4 28 button head bolts to hold the assembly together, when I did get the threaded rod to actually hold it and throttle to clear, the tension caused the bolts to come loose from the spacers and it sucked a spacer into the throttle blades. This happened at end of a dyno pull, luckily it was just a brief scare when thottle hung when letting off and it DIDNT get sucked into engine and into a valve, I highly advise people to stud those spacers with red loctite and put ;locknuts on OUTSIDE of FA or at least red loctite the screws gaffrig has in there, Smitty

I was wonting to see where i could source the volute used on the throttle body or do ypu offer them for sale and a picture of them? i am in the process of building 540 and with pro flo xt intake that has 92mm throttle body using holley hp efi.

Do you have a Proflow XT intake already? I spoke to Holley and they have absolutely NO CLUE when they will be producing and shipping new orders. Its been 2+ years since one has shipped to a customer!

My plan is your plan almost exactly but looks like I am going to either adapt Holley HP to the stock MPI intake or go with multi-port throttle body style injection system.