Gas tank fuel problem

[Def30]

I recently discovered a really bad rust problem in my 91 318i Convertible. A local guy can sell me his gas tank out of an 89 325is. My question is will his gas tank fit on my vert? I just cant tell if my tank is a 55L or 63L and same thing with his....
I have done at least an hours worth of searching, and i'm still a bit confused with the interchange system.

[Def30]

I forgot to include, according to real oem they are both 63L. Does this mean they are gonna interchange for sure?

[Quadrajet]

As I recall the 325i's have 2 tank senders. Check and see if you have a sender on the driver's side of your tank. Have you checked realoem.com yet?

[Def30]

In case somebody searches this later down the road.... all late model gas tanks interchange as long as they are the same size (based off of my experience). The new tank was a 63L just like mine, it went right in, but it took a few curse words and a lot of laying on your back with crap getting into your eyes.... lol.