"Fuel cell bladders do not have to be replaced every five (5) years.
The answer on that topic is the typical "it depends.""
My "bladder" is an FS Enduro unit from 2001.
Never replaced and no deterioration.
I replaced the foam after about 13 years, even though it was fine.
Most of the time I used high octane (usually 110) leaded fuel from a small racing fuel company in my area.
So it depends.
https://fuelsafe.com/rb-ed./
Yes a rectangular cell can be fitted to a '75 911.
It is a race car!