The G34 is the same as a Glock 17 frame. The frame is too long for the slide. Also I believe the locking block is different too.

You may be able to get the G34 slide to fit the G19 frame though, after swapping a couple parts out. Idk.

 

wondering if a G19 slide will fit the frame the G34 comes with?

Negatory, Ghostrider.

 

No

Don't do it.

In 9mm the slides frames of the following can be mixed and matched:

G17, G17L, G34

G19 cannot be put in the mix.