These issues have been talked to death over in the P15-D-24 web site. Also, on the HAMB.
The flathead 6 engine will produce more power by doing the following in order of the increase:
1. Increase cubic inches (251 or 261 engine)
2. Increase the Compression ratio. MAX of 9 to 1 or you will have head gaskets go on a regular basis. Best street compression is 8 to 1 for longevity and gas. If you go past 7.5 to 1 you MUST deck the block and head or you will have sealing problems.
3. Open up the exhaust. You will see a big jump if you put a set of split headers on it. It does not need to be duel headers all the way back just down to the bottom of the flange.
4. Carburetion. A small increase will help. BUT, it must be calculated by the pumping of the engine using CR, actual cubic inches, and a very low VE ratio. At the most you will want a 2-bbl. Two very small 2-bbls will work but unless you jet them correct you will just waste gas. Use progressive 2-bbls only.
Now all the above is on the assumption that you are using a stock cam. You can get a more radical cam from http://www.edgyspeedshop.com
. He also knows more about making speed on these engines than anyone.
On my 1947 Desoto Suburban at 5000 Pounds I rebuilt the engine to the following specs:
251 Cubic inch + 20 Over.
NOS Crankshaft 0-0
about 9 to 1 CR.
New (not reground) stock cam shaft. (I wanted the ramp angle to not be so radical as you get with a regrind. Minor point but can be important at sustained high RPM's)
New head that was CC to +/- one CC on all cylinders and polished to help with carbon build up and hot spots. Plus, makes car idle real smooth.
Everything balanced well.
Connecting Rods modified to use ARP Ford 351C rod bolts.
ARP main and head studs
All new lifters and valves.
All new time chain and gears.
Stock exhaust and carburetion.
I used an original hot rod aluminum Duel Intake and it was too fussy for daily use. The single runs just as strong. If anything, I would put headers on long before messing with the carburetion.
I use fluid drive, a 3 speed stick, and BW overdrive with either 4.11 gears or 3.91 gears.
Been running strong for years. My 2 cents worth.