I did exactly this because I didn't listen to the advice given on this board, I gained a few MPH and have zero hours but that's it. If I could do it over again I would purchase a 500 hp custom motor from a builder on here.
The small jump in horsepower from a stock 454 to a stock 502 is not worth the money or effort.
If you have a chance to work with a reputable builder (not an internet engine company) you will be building a relationship going forward which can be very valuable for the long term

Depends... Warranty is only a word unless they are willing to stand behind it

This is why I'm researching repowering now. So when it comes time, I'll know which way to go.

I went with Reher-Morrison power in my race car (980hp 565 BBC). Did it right the first time with the race car. Doing my research now in order to get it right the first time with my boat as well. I appreciate all the input, guys. Thank you!

Yup. A good amount of other people have too. On new motors 'GM Marine' engines also.Check here and other offshore forums. You'll find the crap.

Its not a small jump in CID first of all. If you have a Gen v or 6 454 and its a mag. take the 4.000 stroke crank, 7/16 rods, oil pan and get a 502 block. build a 510-515 CID engine, put some Dart Iron Eagles on it or buy some AFRs and a cam from Madara. New pistons, resize rods new bolts and build a 630 hp package with a carb or EFI. You just have to do it with the correct parts and the right person doing it for you.
I took some 502 500 HPs that were shot!! Lifters had wheels falling out as they came out, intake off a car that was at a angle, threw the heads away and used a set of Iron Eagles, that were new, that someone else completely cobbed up and had to install new valves, valve job and spent time working the chambers and bowls. Called my cam Guru, Madara up and we set up a hydraulic roller. These engines were put in a 41" Sonic, 606 lbs torque@ 4400 / 631 hp@ 6000, think I used a Holley 900 cfm, Daytona Sensors control box and the Thunderbolt distributor. Real simple and cost effective.

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This is a dyno sheet from the build.

Great info and got damn great sheet !

Power is obviously great for what the motor comprises of, but, more importantly to me is.....

You know how long it's been since I've seen and honest and 'stingy' dyno sheet posted on the internet ?

You gave the customer what they deserve - a good motor for their $$$$ and the truth of what it will do for them.

Thanks and keep doing what you are doing.

SB, I really appreciate; your comment.
I myself; would always like to build a package with high quality, top shelf parts, who doesn't? It's like Christmas, opening up boxes, not having to deal with dirt, rust and the clean-up. It comes down to the experience level, what will work and what parts will jeopardize dependability. There's something nobody wants to hear; hey your stuff is junk and you need to throw it in the trash. Some clients want everything brand new as long as it's not a waste of money, peace of mind along with some type of warranty. All of us work hard, for money and gratification of accomplishment, if it wasn't for the accomplishment, aspect I would probably be out of this business.
This includes customer relations, mostly which I refer to as friends. Guys like; Madara and Haxby which we speak almost on a daily basis. Some of the OSO members have called me on the phone for help at times. Borgie and I have conversed numerous times. You kind of get to know people, even if you have never met face to face. Just think if it wasn't for the internet, what would we be doing today discussing boats?

Search for the ZZ502 threads on here. Good quality motor that needs very little to go boating. Long block from about $7600

That is definitely a good option.