I'm unsure as to whether or not you have the complete question posted, but I will work with what you gave.
It is possible to say that the population density of a town is 145 people per block. It can depend on how many people live in each house, if there are any vacant houses, how many houses are on each block, and if there are any apartments on the blocks. Without certain details concerning them, one can only assume, but from the details that I have it is certainly possible to have 145 people living on each block.
It <em>does</em> make sense to say the population density of a town is 145 people per block because the formula for population density = # of people/unit area. "145 people per block" is the same thing as 145 people/block, which matches the formula.
