Those 70's models truck carbs can sometimes really throw you off, they did a HORRIBLE job of date stamping them from the factory.
Looks like you are missing a number at the very front of the TDJA, The T probably stands for the truck line which makes perfect sense since that 1.21 carb in the 70's was one of the biggest carb at the time and those were all installed on the trucks. The DJA would be a model type, but without the year code in front would be hard to look up.
Bottom line is the 1.21 is the venturi size indicating its a 351 CFM carb, remember though 2 barrels are rated differently than the 4 barrels. Usually this carb would have been on a 390 engine, So it might be a bit much carb for your 351. But the next size down is a 1.14 which is a bit less than you need for the 351.
I am also not a big fan of the 2150's as well especially sine the 2100's are readily available. The 2150's with thier emmission aditions can sometimes cause problems, Often thats why these carbs were removed, and you often get someones problem child.
Hope that helps you out,
Bill White
White Automotive
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HAVE A NICE DAY!!!!