No, you don't have to loosen *all* the main bearing caps to change the crank seals on a 1980 Chevy 350 small-block. You only need to remove the caps that directly access the seals. This is typically the front and rear main bearing caps. However, it's often easier to remove the harmonic balancer and the timing cover to get good access to the front seal, and to remove the oil pan to access the rear seal.
While leaving other caps in place saves time and reduces the risk of misalignment, some mechanics prefer to remove all caps for easier access and cleaning. However this is not strictly necessary.
