I would like to see a picture of your El Plymouth. It sounds interesting. I am in the process of installing a 251 CI big block flat 6 into my 1948 Plymouth. My engine is bored .030” over and I have installed Chevy stove bolt pistons. I also have installed Chevy SS valves. I sent my cam to Earl Edgerton in California and had it reground to 260 duration @ .410” lift both intake and exhaust. I sent my manifold to George Asche in Pennsylvania and he split the exhaust and added dual carburetors. I set up a test bench and have run my engine several hours. Just last week I dropped it into the car. I had to do some extensive modifications to the front saddle and to the radiator bracket. But it now fits like a glove. I have completed modifications to my bell housing to accept a T-5. I have completely rebuilt the transmission and also installed a pilot bearing as opposed to a bushing. I have rebuilt a Dodge 8 ¼” differential with 3.55/1 gears and it is installed in the car. There are lots of avenues to pursue but in my opinion I like keeping the flat six. If you need any details on how I have done any of this let me know.
Don Coatney
