A large software system exists in different forms, as different variants targeting different business needs and users. This kind of systems is provided as a set of "independent" products and not as a "single-whole". Developers use ad-hoc mechanisms to manage variability. We defend a vision of software development where we consider an SPL architecture starting from which the architecture of each variant can be derived before its implementation. Indeed, each derived variant can have its own life. In this paper, we propose a novel approach for Software Architecture Product Line (SAPL) Engineering. It consists of, i) a generic process for recovering an SAPL model which is a product line of "software architectures" from large-sized variants. ii) a forward-engineering process that uses the recovered SAPL to derive new customized software architecture variants. The approach is firstly experimented on thirteen Eclipse variants to create a new SAPL. Then, an intensive evaluation is conducted using an existing benchmark which is also based on Eclipse IDE. Our results showed that we can accurately reconstruct such an SAPL and derive effectively pertinent variants. Our study provides insights that recovering SAPL and then deriving software architectures offers good documentation to understand the software before changing it.