I run the HP500 600SC sea pump with attached fuel pumps on my 547's, one is going, losing pressure after 3000 rpm, what high performance pumps also fit? its just a 454 style pump right???

There's an oil reservoir that the fuel pump cam rides on. The oil tends to get very diluted with gas and wears out the arm/lobe. A standard big block pump has a different arm. Hardin makes a nice replacement

So it is different, good thing I asked....

Anyone know part number for stock one? or Hardin one?

if your arm is ok order any of the 2 piece pumps from summit or jegs and use your top half with the new bottom. Done! High flow pump that fits merc. I have 2 that I did the swap on I'm gonna be selling.

I will look into that too... just was hoping to grab one and put it on, but maybe I will swap stuff around..

Merc part number is 861677T or 818383T I believe the second is the "discontinued" part number and is super seceded by the first number. A quick eBay search they're about 105$. I have a carb hp500 motor with the fuel/seawater pump and this was the part I used when replacing mine. Has worked flawlessly for 40+ hours now.

whats different on the arms?

I have a new stock one from my 454's I broke the base off body when engines shifted in bed of the truck, can I just put the round part on my bad body?

It's the length and shape of the arm that's different. The 454 crank driven fuel pumps the arm is much longer and has a more pronounced curve. The sea water fuel pump has a shorter arm and it's alot more straight. As far as volume I believe they push the same. And if the arm is worn on your pump from lack of oil or diluted oil you have to replace the top half. I don't think the diaphragms are replaceable. .