It is rare for a glow plug to fail. It is far more common for someone to have mis wired them. Some are series wired and others parallel and most don't understand the difference.
If the new ones clearly work better then great.
Dont't confuse the heating time of older GP's with that or modern ones. You cannot overheat the old ones as the resitance increases with temp and the current drops so it is a self limiting situation.
If you have not bought them yet, then I would suggest you post photo of the plug and of the meter and its scale you used to check them.
Readings are usually 0.95 ohms. It the GP's have a pencil tip then you cannot touch the heating element
If you have a wiring diagram, post it.
Post a photo of the side of your engine with the GP's
Dave
M7040