First if you have any plans to keep the flathead let me say you can have a very reliable, decent performing engine that you will not see a 100 of at the next car show. The flatty will draw attention when you have the hood open. You will not win many drag races with the flatty, but I am very happy with mine. I had a 383 / 727 in my last 48 Plymouth coupe which ran like a scalded cat. I now have a 230 flathead and while it obviously does not run like the V8, I am pleased with what I have as I am a different kind of car guy than I was 30 years ago.
If you want a little more Umph out of your flathead try to find a 230 out of 50's Plymouth or Dodge. I recently made the change which was a bolt in and the performance difference is definately noticible. Drove the 218 as my daily for 4 years in Dallas and it did just fine but the 230 was an improvement.
You need to check out the P15-D24 Site
Once there you will find a ton of info on the Mopar flatheads. Also search for "Olddaddy" he sells a pretty much bolt on disc brake kit for the old Mopars as well as a front shock relocation kit. The old Plymouth has a good front suspension once the shocks are relocated.
For Speed Parts
You can look at Edgy Speed Shop
for finned aluminum head, intake and reground cams
Langdon's Stovebolt Eng Co
for intake, cast iron headers and HEI distributor
Another thing you will want to look for is an Overdrive Transmission. There are several options a stock Borg Warner R10 OD from a mid to late Plymouth or Dodge, a T5 from an S10 or some of the later Mopar 5 speeds.
for an adaptor to mount a T5 5 speed behind the flathead
Quality Engineered Components
for the late model Mopar 5 speeds or automatic behind the flathead.
The R10 is a bolt in if you have the standard 3 speed now. All you would have to do is add a couple wires to control the Solenoid / Govenor.
Alternatives for the OD transmission is a rearend swap to an 8 3/4 from a late 60's - early 70's Mopar B Body or a 89-90 Dakota rear with something like a 3.55 gear. Your stock rear is either a 3.90 or a 4.10 so the taller gear will help on the highway. Once you relocate the spring perches this is pretty much a bolt in.
For Standard Rebuild Parts
Terrill Machine 254-893-2610 (TX)
Vintage Power Wagons
Roberts Motor Parts
Some Other Interesting Reading
Blueskies 50 Plymouth Build
Interesting HAMB (Oh what a dirty word) Threads
Hot rodded Mopar flathead 6's here?
Flat head 6's lets see them
218 Performance bolt ons?
Projects - Rebuilding the Plymouth Flathead 6
Here are a couple pictures of my 57 Plymouth 230 in my 48 Plymouth Coupe. On my Dallas to Joplin round trip for the HAMB Drags I got 20.9mpg out of it with the R10 O/D transmission running 65-70 MPH on the highway.
I do not have any video of the 230 running yet but I do my old 218 and I think it sound pretty good. 218 Running