for Buick, the engine code is a two letter code which tells you year of production, displacement and type of application ( 2 barrel or 4, etc ).
in the code 'PD', 'P' is the year code and denotes 1968. 'D' codes all the other information and tells you that it's a 430ci 4 barrel engine rated at 360 hp. this would originally have been from an Electra or one of the other big cars.
on the other hand, if it's supposed to be a 'PR' code, that's a 1968 400ci with 4 barrel carb that would have had to come from a Gran Sport or, less likely, a Sportwagon and would have been rated at 340 hp.
remember that there isn't any difference between the 400 and 430 externally. the differences are bore size ( 4.1875" vs 4.0400" ), cam and different carb jetting. so most of the castings can be swapped between the 400 and 430 engines.
considering that you've found a "400" on the block, my expectation would be that it actually is a 400 cubic inch.
the EIN stamping would provide more clues.
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