The '47 DeSoto uses "Floating Power" mounts which is similar to a motor plate that mounts on the front cover, the difference is the weight of the DeSoto six is suspended on heavy rubber fabric. The actual mounts are a large inverted "U" with the feet sitting on the car's frame, the front timing gear cover is accessible thru the center of the "U", the water pump is located on the top of metal arch of the Floating Power system. This leaves the entire sides of the motor free of mounts, everything is very accessible on these motors.
The 265 is the motor I'd like to use, except for the crankshaft and connecting rods it is identical to the 236 flathead six that is in the '47 DeSoto. I've been told that a good place to find a 265 is in Massey Harris farm machinery as Chrysler sourced its motors to M-H. The 265 six were installed in cars during the early 'Hemi' era, some were used in mid-sized trucks.
Unfortunately for the flathead the development of speed parts ended with the introduction of the V8. Most focus was on the smaller and lighter Dodge and Plymouth motors. The big six didn't get a lot of performance parts development. For the most part the speed parts of the small Mopar six are limited to various high compression alloy heads, intake manifolds, and reground stock camshaft.
A relatively unexplored option on the L-head motors is turbo charging. The '47 motor has 6.6:1 CR, by the late '50s these motors were running 8.5:1 CR. On paper a '59 230 Mopar L-head six has about the same horsepower and torque as a 225, but the 225 has a broader operating range which makes this motor feel much more responsive. The old motors are strong enough to handle turbo charging, but few people have attempted this. Most high performance build ups on the Mopar flatheads use nostalgia parts with designs more than sixty years old. The crankshaft and bottom end of the flatheads are similar to a slant six, but they can benefits from cross-drilling the journals for better oil flow. The mythology of these motors is that they will throw a rod when you spin them too hard because they have "soft" crankshafts. The Mopar literature states that the cranks are forged high strength steel, without any mention of the specific alloy used. I believe minor improvements in the oil system can allow these motors to live at higher RPMs. But without better breathing the L-head won't make much power at high RPMs (relative to a slant six). I believe a turbo could really help one of these motors, make gobs of torque under 3,000 RPM.
Another thing to do is give the motor better transmission gearing. The narrow, but deep torque band needs a five speed. Most five speeds have an O/D, which is an added beneift as a 3.9:1 axle to 4:10:1 was common in cars of this era.
I thnk you can make the L-head perform, but you'll be on your own for a lot of the mods.
The 236 DeSoto has 8 inch rods, swapping in the 265 crank gives you a half-inch increase in stroke(lots of torque). I have looked for modern pistons to fit ths combination, someone has said that Chevy 194 six pistons will work with the 8" rods and 265 crank combo, but it requires a .150 overbore. I'm afraid of head gasket problems with that much overbore.
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. Wow ! I was hoping it was fairly straight forward swap. I know 1959 and 1960 Plymouths and Dodges used the same body & frame those two years even though '59 was the last year for the flat head six and '60 was the first year for the slant six - I was hoping maybe motor mounts were the only major change.
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. Will ya'll still speak to me if I tell yens I've got a 1964 Fairlane under the carport