The speculation got kicked into high gear Wednesday after SG posted a photo of herself reading Linda's 2013 memoir, 'Simple Dreams' ... without much context or anything at all. That was enough to get folks to wonder if they were collaborating, or if Selena was teasing a future acting gig. Mind you, this was around the same time she said she was taking another break from social media.

Anyway, sources with direct knowledge tell TMZ that Selena has, in fact, been tapped to portray Linda in a forthcoming biopic -- although, we're told it's in the very early stages ... and the rest of the deets about other cast members and a studio being attached are hazy.

With that said ... what we do know is that Linda personally approved of Selena herself, and we're told she's happy that SG is attached to the project -- feeling like she's the perfect fit. In fact, our sources say Selena actually met with Linda a couple months ago at her home where they spoke privately.

We're told Linda and her team are directly tied to the production -- so they'll oversee this ... with our sources noting the plot will revolve around her book ... which was optioned.

Of course, people are losing their minds about this news for a couple different reasons -- one, Selena certainly feels like great casting for something like this ... 'cause she does have a passing resemblance to Linda. Plus, they're both of Mexican descent -- so there's that.

Naturally, there's also the singing element -- Linda's got a unique, robust voice ... and Selena is obviously a trained professional singer herself. So technically, she can pull this off.

With Selena's casting, ya gotta figure this is gonna be a splashy film with some major talent ... and with biopics like this in the recent past -- there may even be awards buzz around it.