there are, or were, two stampings on the block that would designate year.
the two letter engine code for a 1970 Electra should have been "SF" and indicates the 370hp variant, which was the highest OEM rating Buick ever had in a production vehicle.
the EIN ( short variant of the VIN ) should be something like "4 0 H xxxxxx". 4 indicating the Buick division, 0 indicating the 1970 model year of production, H indicating the Flint MI plant ( there are other plants but Flint is primary ) and the last 6 digits matching the VIN of the original car it was installed in.