No, they are not interchangeable grades and what is the difference? Have you even a copy of the referenced ASTM A 320 material specification? If not get a copy and you will see the obvious differences between a L7 quenched and tempered bolt grade and B8 austenitic stainless steel bolt grade.
Of course they are not interchangeable. Both L7 and B8 bolts are applicable to low-temperature service. However, L7 is a Cr-Mo low alloy steel while B8 is a stainless steel(SS 304). L7 is recommended for temperatures above -73℃ while B8 for temperatures above -255℃(such as Liquid Natural Gas and Liquid Nitrogen services).