Have you measured the barrels for wall thickness? if they measure fine shoot it and have a great time.
Lengthening chambers from 2 1/2" to 2 3/4" happens all the time. That does not make it unsafe to shoot.
In the UK that would make it out of proof and would have to go through re-proof for the new chamber length.
As long as wall thickness is OK you are fine. Enjoy


Mike Proctor