I have an 85 vt500 mate and as for another tank, chopping it to fit the backbone of your shadow is pretty much your best option.
I have an 86 VT500. Have yet to find a different tank to work with this bullshit double spine frame. I have seen some people cut the top frame and convert it to a single spine. At this point I just say work with what you got. Sometime this winter I'll just metal-flake my tank and make the best of it. Fuck it.
I dished mine. It looked better but it just didn't look right for the overall flow of my bike. I will try and find a pic but It just looked like a shadow trying to look like a chopper with a cafe tank.
I just got a tank from http://tommyjeanbrutalcustoms.blogspot.com/ He knows his shit about shadows.
I guess it all depends on your definition of cheap. There's a few of them floating around this forum. They look slick I am happy I spent the cash and got what I wanted.
I have an 84 honda ascot vtwin 500 then an 88 shadow 600, then a 2001 shadow spirit, since you have a big motor, your best shot is to cut away all the back bone crap you have now, and make it a one piece back bone that will accept almost any cool fuel tank, at least thaty is what i would do, I would love to have an 1100, whether it be a four cylinder or a v twin, and i would make it the way it would accept what is necessary to make it right, for you have a very powerful motor there, and a really fast chopper is very cool, so far i have sohc cb750 choppers as well, they are pretty fast, so, I could only imagine what it would be like to have an 1100 motor in a rigid frame with a very long front end as well, then, with alll of my tooled leather werx, it would be quite fun, and unique, since you don't need an oil tank a drop seat frame would be great for that motor as well, that way with a really long front end that sits high with bars that come down for you to reach without lifting your hands above your shoulders, you would be looking through the frame and forks to get around, i have one chopper that is almost that extreme, I do however get to see the street infront of me about 20 yards in front of me before that, I am looking through my frame and forks, and i really like that, most of the bugs hit the frame and forks before they hit me, I usually feel the moisture they splat off. So, consider a drop seat etc, I know, most people do not want to go extreme, but, once you do, it is all over, that is what you will prefer, it is pretty cool answering peoples questions, when they ask, is that legal? and is that hard to ride? I love the first one, it is the most asked, and i always, always, reply, no! it is not legal, totally illegal, just the way i like it! they shut up, and walk away, I like that also, when they shut up and walk away
If you really want to know how to do a whole new back bone replacement, just start by cutting out the whole back bone assembly you have now, and clean up the neck area, and start by adding in a single tube as your back bone, and gusset the heck out of it, at least this is what I do, but, usually I have the neck completely off so I can clean it up better, and place a plate on the back of the neck to give it a flat surface, all the while completely welded up to make it all that much stronger, For me, it is easier to actually do this rather than explain how to do it, I always go the extra mile and weld everything very solidly, never a spot weld, always welded solid on all sides to make it a more solid unit, if you were closer i would assist you in doing this here at my place to make it more solid, and really this way it would only take about an hour and a half, that is as long as we have all the materials needed to make such a difference in that type of frame work, I would also have you do all the running back and forth and such as I cannot hardly walk, let alone run is never an option for me, due to my accident that has left me permanently disabled, yet, I still ride and do lots of welding, and other things that create what I want, it is funny to see me working in the garage, I slither like a snake to get back and forth while welding, it really looks very strange, but, it gets it all done