The model year starts in June of the preceding year... A 12-11-68 date would be a mid-year production 69 model... In Dec of 68 they were making 69 model year cars....
That works with the engine number. I'm guessing now the L was probably an E. After 340 there may be an R or P and then 2750 0156, making it a '69.
I'll have to go look for the VIN on the pan rail or the bellhousing rail.