To answer the last question first, yes, the plastic tank is a fairly easy replacement, provided you can find one. In fact, i came across many of the front plastic tanks over the years, but not so many of the rear.
If these tanks are metal and rusty, I remember years ago there used to be some method of coating/lining the inside of the tank; you might try a google search of it.
The coffee can is just a vacuum resevoir; if it holds vaccum, no problem there.
I'm a little surprised that you couldn't remove the bolt (though I must admit that I've used a little 'love' from a torch with varying sucess); but, I understand some problems are just a pain.
Let me know how it goes!

 

My new intake arrived today. EGR diaphragm is torn so I will replace as well. Coffee can replaced......... tomorrow night will tell it all.