I think some/most of this information/interpretation is also posted above:
3733950 : 1955-2nd through 1957 261 block casting code
3836850 : 261 head casting code used all years
What is the complete code pressed into the block just to the rear of where the distributor goes into the block (can you post a link to a photo)?
Also,
what are the block date casting code and the head date casting code?
"sc425 and maybe another 5. thi can see pressed into the block. also that C could be a 0 hard to tell"sc425 maybe sc4245 maybe S04245 maybe S04245S maybe F0425S
S 04 25 5?
S 04 25 S?
F = Flint, MI
04 = April
25 = 25th
S = big truck with 261
??
Your water pump is a 54/55st style (it extends above the top of the block), but it might have a relocation plate?
It is possible you have a 1963+ replacement block/head. If it is, it is the third I have seen (heard-of). Maybe Greg_H will chime in - WD Foster gave him information about these.
If it is a replacement block/head, the date casting codes might be interesting. And, the engine serial number is a "replacement" code that might have been stamped incorrectly/oddly.
This might be a guessing game.
