Ron,
Sorry for the delay in getting an answer back to you. I had to go find and dig out my pocket sized resource book. Jerry is correct in the info that the "casting code" # 3921968 could indicate either a 230 or 250 engine from 1967-1979. The key to which engine you have is your stamped engine number.
According to my resource book "Chevy Inline 6 Engines 1929-1989" by Mark S. Allen (Edition Copyright 2008 and last updated April 2008), your stamped engine number/letter sequence decodes as:
F = Flint assembly plant
10 = November
16 = 16th day of November
PF = 1968 C/K-10 or 20 truck - 250CI, 155HP equipped with a 4-speed trans and RPO K19 Heavy Duty Clutch*
*NOTE: the 1968 model year is most likely based on what can be seen in one of your earlier posted pictures as the block casting date codes "CON 2 J _1 7" would indicate it was cast on Conveyor 2 (CON 2) in October (Code Letter J) something (next 2 numbers aren't clear but would indicate day of that month) 1967 (7 standing for the year cast). This would make sense as 1968 model year vehicles were on sale well before the actual start of the actual 1968 calendar year. The "PF" suffix code was also used in 1969, but the likelihood that a block cast in '67 wasn't used until '69 model year is not realistic.
Now that you have identified what you have, you can ask more questions about how you may be able to adapt this engine to your '41 AK.
Suggest you ask questions on adapting or upgrading your '41 AK drive train to use this engine be asked in the
Driveline forum.
Hope this is helpful info.
Dan
