I really don't think they let this block sit for 2 1/2 yrs before they assembled the motor.
I tend to agree with you on that fact. No question something is 'strange' about the casting date of the block compared to the top pad. Since the pad says 'G' that would indicate 1971 and the 1216Hp would have to be 1970 since the production year ran from July 1970-July 1971. The 1972 Model cars started being produced in the summer of 1971 just like todays models start in the summer previous to the model year that are stamped.. So that would have to make the 1216 Dec 16, 1970. I have no answer for the weird difference in pad/casting #'s.
You might want to read this info on Dampers/rods/balancing. Here's some info from the link:
Shown above is a standard 1851535 "RB" USED connecting rod, Chrysler part #'s 1851533, 2406770, 3462426, and 4027096. Commonly referred to as "LY" rods, these were used on the majority of "RB" engines. Weight is about 860 grams.
Rods: Six-Pack & HP engines:
Shown above is a 2951908 connecting rod, commonly known as "six pack" rods. These rods have a thicker, heavier beam cross section. These were used on HP and truck 440's (along with forged cranks only) from 1970 to 1975 under Chrysler part # 2951906. Weight is approx 920 grams.
Damper: standard rods:
This first damper is an early forged crank internal balance type, measuring 7 1/4" diameter by about 7/8" wide, with the offset (pre 72) pulley bolt pattern . Most of these types of dampers will be Chrysler # 2275897 or 2658457. These were available with either the solid ring (above left picture) or the cut ring (above right picture), and the holes in the hub may be either round or square. The difference appears to be based on the supplier Chrysler happened to use, as we have seen both types used for the same year, part #, etc. The cut ring type seems to be more commonly used. These were used from about 1962 until 71 with all forged crank engines. In 72, the bolt pattern was changed to be symmetrical and therefore a different part and associated part # were used (see below.) These usually do not have part numbers cast into them (although sometimes 2658457's will be marked on the rear inside of the hub.) Check for the offset bolt in the pulley bolt pattern to be sure.
Damper: Six Pack Rods:
Shown above is damper #3512017, commonly known as a "six-pack" damper. This damper can be easily identified by a small elliptical weight attached to the center hub. This version was used in '70 and '71 and has the offset pulley bolt pattern. Be careful not to confuse it with 3614372, which was used only in '72 and looks identical except for having a symmetrical pulley bolt pattern. Even though this damper does have an elliptical weight, it was never used with any cast cranks. These dampers were used with steel cranks and "six pack" (2951908 casting #) rods. The extra weight is to offset the added weight of the extra steel in the rods. Another important fact is that even though six pack rods continued to be used up until 1975 in HP and truck 440's with steel cranks, they did NOT use this damper. As you can see from the chart above, any 73-75 steel crank 440 (with either six pack or standard rods) would have come with the 3614371 damper. I have found this to be absolutely true.
Link for above info: 440Source
Most HP engines whether a six-pack or HP engine came with the Heavy rods and forged crank thus they used the externally balanced damper. The six-pack engines came with different pistons compared the Hp engines. So you can have a HP engine externally balanced and still not tell if it came out of a six-pack car. There has been and continues to be many posts on other forums about this very subject. There are always a few stories about different combo's but basically this is the most common info on the '69-71 HP and six pack engines.