Viejazo is a colloquial term derived from "viejo" (old). While it can literally mean "old man" or "big old thing," in the context of this song, it's used to describe something that's old but still powerful and reliable, like a well-maintained, older vehicle.
The lyrics mention "aunque en el gasto se encuentra el viejazo con tantos caballos," implying that despite the expense, an older, powerful vehicle (with "tantos caballos" or many horsepower) is still a valuable asset. It's an interesting word because it carries a sense of respect for something aged but still capable, rather than just being old and worn out.
Aldo Trujillo's "Porta Buenas Botas" is a fun look into the life of a self-made man who's all about family, hard work, and enjoying life on his own terms. He didn't do well in school, saying "no le entró a las clases pues no se le dieron" [he didn't get into classes because they weren't for him], and even "pasó hambre si les soy sincero" [he went hungry if I'm honest]. But that didn't stop him! He's a "hombre ranchero perfecto" [perfect ranch man] who earned his money and now takes care of his brothers.
He's got a real love for his custom cars, which are "arreglados aunque en el gasto se encuentra el viejazo" [fixed up even if the expense is a big one], and his horses. He's proud of his Mexican heritage, wanting to "llevar la cultura de ayer ver sembrando se lleve el orgullo de ser mexicano" [carry the culture of yesterday, seeing it sown, carrying the pride of being Mexican]. He loves to party, with "botellitas pensar el ambiente bailando mis pasos eso me divierte" [little bottles to think about the atmosphere, dancing my steps, that entertains me]. He's found his path, even "aquí en el extranjero" [here in a foreign land], and is living life his way, proving that he can make it happen when no one else believed in him.