I installed 500 EFI cams in my Merc Gen V 454's a few years ago. I don't regret it. Didn't gain much performance, but the roller piece of mind was worth it.

The 168731 cam is close a 500efi cam and they didn't have any reversion issues. The HP500 carb engine cams were different and they had reversion issues at idle.
The 168731 is a roller cam

I thought the 731-ish cam that Merc used caused some reversion issues. Maybe I got that one a little wrong?

Had to dig around for the HP 500 specs... courtesy of RMBuilder a way's back:

284*/292* @ .004"
222*/230* @ .050"
.576"/.598" valve
110* LSA
105* ICL
115* ECL

That 110* LSA probably didn't help much with the Gils that they originally came with.

Have the 169621 500 EFI cams in the Bullet - 060 and 080 over 454s with B&M 250s. 188 heads, nothing special just super reliable and really strong. They really like the 500 EFI cams. Anyway, supercharging isn't the subject here.

Yeah and Merc actually changed the Gil tailpipes from this style to the ones that are taller and have more swoop

Have same exhaust Rookie Pic engine but not same intake ,

Mine is more elevate

I agree with installing new hydraulic roller cam and lifters as Griff has suggested, but just make sure that those Gill Marine exhaust manifolds and tailpipes are a completely dry system especially with that cam being on a 110 or a 112 LSA otherwise you're going to have reversion problems.
If you have a "wet" exhaust system, then you most likely will need a lobe separation angle (LSA) of 114° within the cam specs of the camshaft so as not to revert or have reversion of water back up into the engine.

Also I would stick with the Air-Gap dual plane intake manifolds that you said you already have... They will give you better lower and midrange performance on those 454's especially with a heavier boat like the 37 ft Baja that you have. They would provide great throttle response and work great for your application.
You could even add a phenolic 1" OPEN carburetor spacer to keep the carburetor from getting heat soaked.
and for a hair more performance.
Just my .02 but I think you're on the right track with the advice given by the others.

A 168731 won't have reversion issues with full length wet exhaust tails like the ones in the pic on my 525SC. The water mixed into the exhaust about 2" before the rubber hose connection to the exhaust tips.

Griff, would you happen to know/recall what RPM the 500 HP peeked out at with that 731 cam in it?
Thanks