Re: converting to fuel pump on 1957 johnson seahorse
I'd convert the motor. Its easy, inexpensive, and you can use a very large variety of tanks with your motor.
There is a "sticky thread" here, on how to do it. The "cliff notes version" is that you can use a bypass cover off of a later year 28hp/33hp motor, or you can use the original bypass cover.
If you use the 28hp/33hp bypass cover, the procedure is a simple swap and mounting of a new fuel pump. If, on the other hand, you use the original, you will have to drill out the center "boss" (all three are actually there, but were never drilled) all the way through the cover, and the mounting bosses part way through. Those mounting bosses then need to be tapped for the mounting screws.
One peculiarity of the old covers, is that the mounting/pulse bosses are molded very slightly higher into the bypass cover, than is true on the newer ones. Because of this, the Throttle Control Rod (steel rod, port side of motor) will hit the top of the fuel pump if it is mounted right side up. The fix is easy, just mount the fuel pump upside down.
If you make the conversion, don't forget to remove the nipple on the lower portion of the intake manifold, which is the source of pressure for the hose/pressure tank. You will have to plug it will a small bolt of the appropriate size, thread count and length ..... do not use a screw that is too long because it will hit the Leaf Plate, located behind the intake cover (internal), if you do!