As someone in his mid-40s with all kinds of knowledge about drug culture references, the latter doesn't make much sense. Calling Quaaludes "624s" WAS indeed quite common street slang. However, on the streets LSD commonly would have been called either "LSD" or "acid". LSD-25 comes from the notes of the inventor Albert Hofmann. He was working at the time on synthesizing derivatives of lysergic acid. (In German lysergic acid is "LS", because in German acid is "saure".) The 25th derivative he made was what we call now LSD. The average street user wasn't aware of this history, and thus the designation LSD-25. People who were going to trip just didn't say "I'm gonna drop some 25 tonight".
And, it seems quite odd to me that anyone with an inkling to take drugs would be deciding between LSD and Quaaludes. LSD is a psychedelic with a speedy quality. Quaaludes are downers that just get the user fucked up. This would be like a druggie thinking "Do I want to pop a bunch of Valium tonight, or should I bang methampetamine instead?" What would make sense is debating whether to do LSD or shrooms.
As such, I'd say taking the word of the guy who wrote the song seems reasonable.