Not an expert but my book says for 1949 DeSoto..the engine is a 236.6
1951 was first year for the 250.6 for DeSoto which was by the way largest 6 in the DeSoto
However..pleae note that if your engine number does have an A or B or both A and B you have a "special standard" engine..
A equals .020 larger bore and B equals .010 under standard journals.
As for the Spitfire head..looks like a swap in earlier life...
Now IF the entire engine is changed... say from a Chrysler..pre 52 would be 250 and 52 on could be a 265..the 265 block should start with C52, C60-1, C60-2 or C62 from a 53 model...from then on it was V8
post your block number as found on the engind block boss above the generator...