Howdy,
What do you mean by "After-market" manifold? Blue Thunder = $$$ little for no additional gains in a stock engine.
I had a 57 ford as a kid and I switched from a 2 bbl to a four bbl carb and I don't remember the mechanical pump causing a problem. Also, I had (and still have) a mechanical pump in my boat.
The diameter of the diaphram in both pumps (460 boat engine and 292) are about the same. I used to run the boat at 4000 frequently and I never had a problem with the mechanical pump.
I never had a problem with the mechanical pump in my 57 Ford either.
Now there are other reasons to go to an electric pump in the boat. You get fuel pressure as soon as you hit the starter. (wire it to the starter switch AND an oil pressure switch....you DO NOT want it wired to the ignition)
I don't think there's much advantage in a stock engine though.
I am putting a 292 in my F-600. I'll probably use the mechanical pump. I did put a 4bbl carb on it (although it's probably not needed) The 2bbl Ford and Holley carbs work just fine. The Y-guys here can tell you more but your 56 will sound MUCH MUCH better with a "Y" in it.
Go to
http://www.ford-y-block.com/index.html for a lot of info on the "Y"
Also go to
http://www.y-blocksforever.com/forums/Forum3-1.aspx
for a lot of experts!
Fordomatic? my 57 had one and it was fine. If I was going to restore a car I would go to
http://www.transmissionadapters.com/ford_y_block.htm and maybe put an AOD in it to get the overdrive.
Cheers,
Rick