Shopping for fuel pumps, kind of new at this. I know enough to know that you dont wan't too little or too much pressure, but thats about it. I've got a 502 415hp marine long block, with 750 holley, and merlin dual plane intake. I need a fuel pump that mounts on top of the sea pump, like how the 500HP are set up. Does it matter which one or can I just get the same fuel pump merc uses on their 500HP's? I didn' know if that would be too much fuel pressure or not. Are there even any other options besides merc for the sea pump driven fuel pump?

That pulley fuel pump is not very good, go with an Aeromotive Marine electric fuel pump and an adjustable regulator.

Really? What's wrong with them?

I put a KE billet bottom on my OEM Merc top, for the seapump mounted deal. It works very well, no regulator needed. Oh, and -8 lines, and I think CP Performance sells the whole pump if you want a complete set up. As far as fuel pumps, there are a ton of choices, this set up works for me.

I had the seawater/fuel pumps feeding my Holley blues and then to my carbs. The seawater/fuel pumps needed to be rebuilt every year and some times probably during the middle of the season. I wiped out the cam lobe that runs the armature twice. (leaks fuel into the oil fill reservoir). I then went to just Holley blues, I now had to mount the Holleys a little below the fuel pick and works great. I now am going to change to Mallory marine 140, the same pump that I used on the dyno.
just my $0.02

This is what I use on applications such as yours. Call C.V. Products in Thomasville, NC ask for Dan Kent, tell him Mark from MER sent you , and that you want there Billet marine pump for a Merc water pump mount with the fuel press you want. It's 100% rebuild but, it's going to set you back almost $400.00

The pump is nickel plated, have vent line fitting ARP bolts and washers. You'll never have to replace that pump, if you want to switch it to a Gen IV just order the pump arm.

Thanks !

What psi should I be looking at for this engine setup? GPH?

max 7-8 PSI !!!!!!

I run the Aeromotive Marine SS PN 027-11211, runs 140+ gph and is internally regulated to 7psi. Great pump, not too loud and doesn't get hot. I have always had drama's with mechanical pumps, besides, you can't turn the damn things off if you ever need to! I like having a kill switch for the pump as well as ignition.