Could a Glock body (with an extended mag) withstand full auto action?
I do not know.
From the Glock Website:
G18 – Fully automatic 9mm firepower
As a true fully automatic pistol the G18 is one of the most specialized GLOCK pistols. With an external fire selector at the rear of the slide you can switch from semi-automatic to full-automatic firing mode.
It comes with an extended 19 round magazine and can provide greater firepower with the optional 33-round magazine. The rate of fire in full-automatic-mode is approximately 20 rounds per second.
Released in 1987 the pistol has the same characteristics like other previous models and the frame size of the service pistol classic G17.