[bruceman1968]

Hi Bill, you've pretty much got this figured out. But here are some details as well as a way to be sure on the year.
First your casting number 3999289 is for a 1972 to 1978 454 truck motor.
Next your engine ID number T0106TRK decodes as
T = Tonawanda Engine plant
Assembly date
01 = January
06 = 6th day of January
Application code
TRK = TRK was used in 1973,1978 and 1981. Your casting number(3999289) is for a 1972-78, so we can rule out 1981.
Meaning your engine is either a 1973 or 1978 454. The T in TRK tells us that it is a truck motor.
So now were down to whether your engine is a 1973 454 truck motor or a 1978 454 truck motor.
Next is your date code and here's where the confusion comes in. Everything Trevor told you in post #8 is correct. In post #7 you listed a date code of T 01M3. There must be a misread there somewhere. Here's why.

A date code should be a letter for the month, then one or two numbers for the date. Then the head of a screw, then one or two numbers for the year.
Example: A13 for January 1st, 1973 or A173 also for January 1st, 1973. January 11th 1973 would be A113 or A1173

The first digit is for the month. January is A, February is B and so on. December is L and you have T which in not possible for a month code. You did leave a space between the T and the O or 0 so there is a possibility that the T is for Tonawanda. Some Tonawanda castings got a T(for Tonawanda), most didn't. I would need to see a pic to know for sure. But that's not going to be necessary.
Next you have a O or 0. Again not possible for a month code(there is no O letter possibility) and not likely for a date code as it would be 01 and Tonawanda just used 1 for the first day of the month. Next we have the M which is really not possible because the only thing that should be in that area is numbers. There is a casting mark(it's actually a casting of the screw head that held the date code plate to the mold core) that should be in that spot. After the screw head or M is your year code. You have stated that it is a 3. That would be for 1973. Meaning your engine is a 1973 454 truck motor. With a application code of TRK = 454ci, 240 hp, Auto trans. But we want to be sure that the 3 is not a 8. 3's and 8's are the most common misread. Especially with casting numbers. So let's be sure it's not a 8. We can check that a different way.
Look at the pad where you found the stamped engine ID number(T0106TRK). If there is no partial VIN stamped in the pad, then the engine is a 1973. If there IS a partial VIN stamped in the pad, then your engine is a 1978. The 1973 Truck blocks did not have partial VIN's stamped on the pad and the 1978 truck blocks Did have a partial VIN stamped on the pad. So just looking for a partial VIN on the engine ID pad will tell you if your block is a 1973 or a 1978.
1973 truck blocks have no partial VIN's stamped on the front pad.
1978 truck blocks do. TRK in 1978 = 454ci, 250 hp, Auto trans

Never mind all that stuff before the very last digit of your date code. It's a mess anyway. T is not possible for a month code, letter O isn't either. 01 wasn't used on a casting date but was used on stamped dates. You said that the M was clearly a M but honestly..No.(smile) Look at it again and see if it could be a cast imprint of a flathead screw. We really don't need any of the digits in front of the last digit in order to get the year anyway. Last digit 3 = 1973 or 8 = 1978.
If you want to post a good pic of the complete date code, I would love to see it, just so we can try to figure out what you got going on there.
Hope this helps and explains a little more about the date codes.
Bruce

EDIT: Trevor, you beat me to it. I took a long time to prepare my post(smile) We really do need to see a pic in order to get the month and day figured out.