I picked up an 86 with a bad motor for basically nothing. Car was from up north so everything has a pretty nice coat of rust on it. The body itself isn't all that bad, it's just everything that's bolted to it!

So I have a friend that will give me his stock gas tank from his 91 but I'm unsure if there's any changes in the stock tanks...? AS in, will the float fit and read correctly or any other problems.

 

I'm about to put a tank off a 91 on a 88. As far as I read, they're all the same.

 

I had a S5 tank in my S4, you only need the larger straps. As the s5 is 7 litres larger. The gauge read fine, so you are good to go.

 

I have been told that you will need the correct straps, since the tanks are different capacity.

I have been told that the S4 fuel pump-tank sender is different physically than the S5, thus you will need the S5 setup for the S5 tank. Plugs and plays after that.

 

I do remember something about wiring now, I think I had to swap the connector, I never swapped senders, I do know that.

 

Been there.
yep..s5 sender apparatus with connector is way different.
take your s4 out and put it in the s5 tank and use the s5 straps.