Anybody swap a slant 6 with a 904 transmission into a 40s plymouth? What all in involved?

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obliquely-inclined state of mind

Keep in mind that the head of the original 218 in the P15 is only about 23 inches long. The bigger Mopar six was used in Canadian models, but its only about two inches longer. The radiator support can be flipped to gain most of the space for the 237-251-265 swap. The slant six has been swapped into a P15. I think it would be relatively easier to use a 318. I have two P15s. My hunch is a Toyota Supra six(7MGE) would work well, I'm not sure about the pan sump, but the length is the same as the 218. I might get run out of town having the Supra motor in a P15, but it is an interesting option, especially since the 5 speed has hydraulic clutch and ought to be simpler. The 225 requires that the firewall to be modified. The original flathead six is a very compact design. I think some late model four cylinders would do well in a P15, but the 225 length is a headache. Consider a 318 if you must use a newer Mopar motor - the steering box clearance is the only concern and that is easy to deal with. A 218/230 has good torque, the reputation of these flatheads as dogs is partly because so many of these flatties keep running with serious wounds. As long as they have oil, they'll run with hardly any compression. My 230 has Chevy 327 valves, an Isky cam, it'll be able to keep up with modern traffic.